.\" Copyright (c) 2013, 2014 by Michael Kerrisk .\" and Copyright (c) 2012, 2014 by Eric W. Biederman .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" .TH USER_NAMESPACES 7 2014-09-21 "Linux" "Linux Programmer's Manual" .SH NAME user_namespaces \- overview of Linux user_namespaces .SH DESCRIPTION For an overview of namespaces, see .BR namespaces (7). User namespaces isolate security-related identifiers and attributes, in particular, user IDs and group IDs (see .BR credentials (7), the root directory, keys (see .BR keyctl (2)), .\" FIXME: This page says very little about the interaction .\" of user namespaces and keys. Add something on this topic. and capabilities (see .BR capabilities (7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace. .\" .\" ============================================================ .\" .SS Nested namespaces, namespace membership User namespaces can be nested; that is, each user namespace\(emexcept the initial ("root") namespace\(emhas a parent user namespace, and can have zero or more child user namespaces. The parent user namespace is the user namespace of the process that creates the user namespace via a call to .BR unshare (2) or .BR clone (2) with the .BR CLONE_NEWUSER flag. The kernel imposes (since version 3.11) a limit of 32 nested levels of .\" commit 8742f229b635bf1c1c84a3dfe5e47c814c20b5c8 user namespaces. .\" FIXME Explain the rationale for this limit. (What is the rationale?) Calls to .BR unshare (2) or .BR clone (2) that would cause this limit to be exceeded fail with the error .BR EUSERS . Each process is a member of exactly one user namespace. A process created via .BR fork (2) or .BR clone (2) without the .BR CLONE_NEWUSER flag is a member of the same user namespace as its parent. A single-threaded process can join another user namespace with .BR setns (2) if it has the .BR CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace. A call to .BR clone (2) or .BR unshare (2) with the .BR CLONE_NEWUSER flag makes the new child process (for .BR clone (2)) or the caller (for .BR unshare (2)) a member of the new user namespace created by the call. .\" .\" ============================================================ .\" .SS Capabilities The child process created by .BR clone (2) with the .BR CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using .BR unshare (2) or joins an existing user namespace using .BR setns (2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of .BR clone (2)) or previous (in the case of .BR unshare (2) and .BR setns (2)) user namespace, even if the new namespace is created or joined by the root user (i.e., a process with user ID 0 in the root namespace). Note that a call to .BR execve (2) will cause a process's capabilities to be recalculated in the usual way (see .BR capabilities (7)), so that usually, unless it has a user ID of 0 within the namespace or the executable file has a nonempty inheritable capabilities mask, it will lose all capabilities. See the discussion of user and group ID mappings, below. A call to .BR clone (2), .BR unshare (2), or .BR setns (2) using the .BR CLONE_NEWUSER flag sets the "securebits" flags (see .BR capabilities (7)) to their default values (all flags disabled) in the child (for .BR clone (2)) or caller (for .BR unshare (2), or .BR setns (2)). Note that because the caller no longer has capabilities in its original user namespace after a call to .BR setns (2), it is not possible for a process to reset its "securebits" flags while retaining its user namespace membership by using a pair of .BR setns (2) calls to move to another user namespace and then return to its original user namespace. Having a capability inside a user namespace permits a process to perform operations (that require privilege) only on resources governed by that namespace. The rules for determining whether or not a process has a capability in a particular user namespace are as follows: .IP 1. 3 A process has a capability inside a user namespace if it is a member of that namespace and it has the capability in its effective capability set. A process can gain capabilities in its effective capability set in various ways. For example, it may execute a set-user-ID program or an executable with associated file capabilities. In addition, a process may gain capabilities via the effect of .BR clone (2), .BR unshare (2), or .BR setns (2), as already described. .\" In the 3.8 sources, see security/commoncap.c::cap_capable(): .IP 2. If a process has a capability in a user namespace, then it has that capability in all child (and further removed descendant) namespaces as well. .IP 3. .\" * The owner of the user namespace in the parent of the .\" * user namespace has all caps. When a user namespace is created, the kernel records the effective user ID of the creating process as being the "owner" of the namespace. .\" (and likewise associates the effective group ID of the creating process .\" with the namespace). A process that resides in the parent of the user namespace .\" See kernel commit 520d9eabce18edfef76a60b7b839d54facafe1f9 for a fix .\" on this point and whose effective user ID matches the owner of the namespace has all capabilities in the namespace. .\" This includes the case where the process executes a set-user-ID .\" program that confers the effective UID of the creator of the namespace. By virtue of the previous rule, this means that the process has all capabilities in all further removed descendant user namespaces as well. .\" .\" ============================================================ .\" .SS Interaction of user namespaces and other types of namespaces Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID, IPC, network, and UTS namespaces can be created with just the .B CAP_SYS_ADMIN capability in the caller's user namespace. When a non-user-namespace is created, it is owned by the user namespace in which the creating process was a member at the time of the creation of the namespace. Actions on the non-user-namespace require capabilities in the corresponding user namespace. If .BR CLONE_NEWUSER is specified along with other .B CLONE_NEW* flags in a single .BR clone (2) or .BR unshare (2) call, the user namespace is guaranteed to be created first, giving the child .RB ( clone (2)) or caller .RB ( unshare (2)) privileges over the remaining namespaces created by the call. Thus, it is possible for an unprivileged caller to specify this combination of flags. When a new IPC, mount, network, PID, or UTS namespace is created via .BR clone (2) or .BR unshare (2), the kernel records the user namespace of the creating process against the new namespace. (This association can't be changed.) When a process in the new namespace subsequently performs privileged operations that operate on global resources isolated by the namespace, the permission checks are performed according to the process's capabilities in the user namespace that the kernel associated with the new namespace. .\" .\" ============================================================ .\" .SS Restrictions on mount namespaces Note the following points with respect to mount namespaces: .IP * 3 A mount namespace has an owner user namespace. A mount namespace whose owner user namespace is different from the owner user namespace of its parent mount namespace is considered a less privileged mount namespace. .IP * When creating a less privileged mount namespace, shared mounts are reduced to slave mounts. This ensures that mappings performed in less privileged mount namespaces will not propagate to more privileged mount namespaces. .IP * .\" FIXME . .\" What does "come as a single unit from more privileged mount" mean? Mounts that come as a single unit from more privileged mount are locked together and may not be separated in a less privileged mount namespace. (The .BR unshare (2) .B CLONE_NEWNS operation brings across all of the mounts from the original mount namespace as a single unit, and recursive mounts that propagate between mount namespaces propagate as a single unit.) .IP * The .BR mount (2) flags .BR MS_RDONLY , .BR MS_NOSUID , .BR MS_NOEXEC , and the "atime" flags .RB ( MS_NOATIME , .BR MS_NODIRATIME , .BR MS_RELATIME) settings become locked .\" commit 9566d6742852c527bf5af38af5cbb878dad75705 .\" Author: Eric W. Biederman .\" Date: Mon Jul 28 17:26:07 2014 -0700 .\" .\" mnt: Correct permission checks in do_remount .\" when propagated from a more privileged to a less privileged mount namespace, and may not be changed in the less privileged mount namespace. .IP * .\" (As of 3.18-rc1 (in Al Viro's 2014-08-30 vfs.git#for-next tree)) A file or directory that is a mount point in one namespace that is not a mount point in another namespace, may be renamed, unlinked, or removed .RB ( rmdir (2)) in the mount namespace in which it is not a mount point (subject to the usual permission checks). .IP Previously, attempting to unlink, rename, or remove a file or directory that was a mount point in another mount namespace would result in the error .BR EBUSY . That behavior had technical problems of enforcement (e.g., for NFS) and permitted denial-of-service attacks against more privileged users. (i.e., preventing individual files from being updated by bind mounting on top of them). .\" .\" ============================================================ .\" .SS User and group ID mappings: uid_map and gid_map When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user namespace. The .IR /proc/[pid]/uid_map and .IR /proc/[pid]/gid_map files (available since Linux 3.5) .\" commit 22d917d80e842829d0ca0a561967d728eb1d6303 expose the mappings for user and group IDs inside the user namespace for the process .IR pid . These files can be read to view the mappings in a user namespace and written to (once) to define the mappings. The description in the following paragraphs explains the details for .IR uid_map ; .IR gid_map is exactly the same, but each instance of "user ID" is replaced by "group ID". The .I uid_map file exposes the mapping of user IDs from the user namespace of the process .IR pid to the user namespace of the process that opened .IR uid_map (but see a qualification to this point below). In other words, processes that are in different user namespaces will potentially see different values when reading from a particular .I uid_map file, depending on the user ID mappings for the user namespaces of the reading processes. Each line in the .I uid_map file specifies a 1-to-1 mapping of a range of contiguous user IDs between two user namespaces. (When a user namespace is first created, this file is empty.) The specification in each line takes the form of three numbers delimited by white space. The first two numbers specify the starting user ID in each of the two user namespaces. The third number specifies the length of the mapped range. In detail, the fields are interpreted as follows: .IP (1) 4 The start of the range of user IDs in the user namespace of the process .IR pid . .IP (2) The start of the range of user IDs to which the user IDs specified by field one map. How field two is interpreted depends on whether the process that opened .I uid_map and the process .IR pid are in the same user namespace, as follows: .RS .IP a) 3 If the two processes are in different user namespaces: field two is the start of a range of user IDs in the user namespace of the process that opened .IR uid_map . .IP b) If the two processes are in the same user namespace: field two is the start of the range of user IDs in the parent user namespace of the process .IR pid . This case enables the opener of .I uid_map (the common case here is opening .IR /proc/self/uid_map ) to see the mapping of user IDs into the user namespace of the process that created this user namespace. .RE .IP (3) The length of the range of user IDs that is mapped between the two user namespaces. .PP System calls that return user IDs (group IDs)\(emfor example, .BR getuid (2), .BR getgid (2), and the credential fields in the structure returned by .BR stat (2)\(emreturn the user ID (group ID) mapped into the caller's user namespace. When a process accesses a file, its user and group IDs are mapped into the initial user namespace for the purpose of permission checking and assigning IDs when creating a file. When a process retrieves file user and group IDs via .BR stat (2), the IDs are mapped in the opposite direction, to produce values relative to the process user and group ID mappings. The initial user namespace has no parent namespace, but, for consistency, the kernel provides dummy user and group ID mapping files for this namespace. Looking at the .I uid_map file .RI ( gid_map is the same) from a shell in the initial namespace shows: .in +4n .nf $ \fBcat /proc/$$/uid_map\fP 0 0 4294967295 .fi .in This mapping tells us that the range starting at user ID 0 in this namespace maps to a range starting at 0 in the (nonexistent) parent namespace, and the length of the range is the largest 32-bit unsigned integer. (This deliberately leaves 4294967295 (the 32-bit signed \-1 value) unmapped. This is deliberate: .IR "(uid_t)\ -\1" is used in several interfaces (e.g., .BR setreuid (2)) as a way to specify "no user ID". Leaving .IR "(uid_t)\ -\1" unmapped and unusable guarantees that there will be no confusion when using these interfaces. .\" .\" ============================================================ .\" .SS Defining user and group ID mappings: writing to uid_map and gid_map .PP After the creation of a new user namespace, the .I uid_map file of .I one of the processes in the namespace may be written to .I once to define the mapping of user IDs in the new user namespace. An attempt to write more than once to a .I uid_map file in a user namespace fails with the error .BR EPERM . Similar rules apply for .I gid_map files. The lines written to .IR uid_map .RI ( gid_map ) must conform to the following rules: .IP * 3 The three fields must be valid numbers, and the last field must be greater than 0. .IP * Lines are terminated by newline characters. .IP * There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the limit is five lines. In addition, the number of bytes written to the file must be less than the system page size, .\" FIXME(Eric): the restriction "less than" rather than "less than or equal" .\" seems strangely arbitrary. Furthermore, the comment does not agree .\" with the code in kernel/user_namespace.c. Which is correct? and the write must be performed at the start of the file (i.e., .BR lseek (2) and .BR pwrite (2) can't be used to write to nonzero offsets in the file). .IP * The range of user IDs (group IDs) specified in each line cannot overlap with the ranges in any other lines. In the initial implementation (Linux 3.8), this requirement was satisfied by a simplistic implementation that imposed the further requirement that the values in both field 1 and field 2 of successive lines must be in ascending numerical order, which prevented some otherwise valid maps from being created. Linux 3.9 and later .\" commit 0bd14b4fd72afd5df41e9fd59f356740f22fceba fix this limitation, allowing any valid set of nonoverlapping maps. .IP * At least one line must be written to the file. .PP Writes that violate the above rules fail with the error .BR EINVAL . In order for a process to write to the .I /proc/[pid]/uid_map .RI ( /proc/[pid]/gid_map ) file, all of the following requirements must be met: .IP 1. 3 The writing process must have the .BR CAP_SETUID .RB ( CAP_SETGID ) capability in the user namespace of the process .IR pid . .IP 2. The writing process must be in either the user namespace of the process .I pid or inside the parent user namespace of the process .IR pid . .IP 3. The mapped user IDs (group IDs) must in turn have a mapping in the parent user namespace. .IP 4. One of the following is true: .RS .IP * 3 The data written to .I uid_map .RI ( gid_map ) consists of a single line that maps the writing process's filesystem user ID (group ID) in the parent user namespace to a user ID (group ID) in the user namespace. The usual case here is that this single line provides a mapping for user ID of the process that created the namespace. .IP * 3 The opening process has the .BR CAP_SETUID .RB ( CAP_SETGID ) capability in the parent user namespace. Thus, a privileged process can make mappings to arbitrary user IDs (group IDs) in the parent user namespace. .RE .PP Writes that violate the above rules fail with the error .BR EPERM . .\" .\" ============================================================ .\" .SS Unmapped user and group IDs .PP There are various places where an unmapped user ID (group ID) may be exposed to user space. For example, the first process in a new user namespace may call .BR getuid () before a user ID mapping has been defined for the namespace. In most such cases, an unmapped user ID is converted .\" from_kuid_munged(), from_kgid_munged() to the overflow user ID (group ID); the default value for the overflow user ID (group ID) is 65534. See the descriptions of .IR /proc/sys/kernel/overflowuid and .IR /proc/sys/kernel/overflowgid in .BR proc (5). The cases where unmapped IDs are mapped in this fashion include system calls that return user IDs .RB ( getuid (2) .BR getgid (2), and similar), credentials passed over a UNIX domain socket, .\" also SO_PEERCRED credentials returned by .BR stat (2), .BR waitid (2), and the System V IPC "ctl" .B IPC_STAT operations, credentials exposed by .IR /proc/PID/status and the files in .IR /proc/sysvipc/* , credentials returned via the .I si_uid field in the .I siginfo_t received with a signal (see .BR sigaction (2)), credentials written to the process accounting file (see .BR acct (5)), and credentials returned with POSIX message queue notifications (see .BR mq_notify (3)). There is one notable case where unmapped user and group IDs are .I not .\" from_kuid(), from_kgid() .\" Also F_GETOWNER_UIDS is an exception converted to the corresponding overflow ID value. When viewing a .I uid_map or .I gid_map file in which there is no mapping for the second field, that field is displayed as 4294967295 (\-1 as an unsigned integer); .\" .\" ============================================================ .\" .SS Set-user-ID and set-group-ID programs .PP When a process inside a user namespace executes a set-user-ID (set-group-ID) program, the process's effective user (group) ID inside the namespace is changed to whatever value is mapped for the user (group) ID of the file. However, if either the user .I or the group ID of the file has no mapping inside the namespace, the set-user-ID (set-group-ID) bit is silently ignored: the new program is executed, but the process's effective user (group) ID is left unchanged. (This mirrors the semantics of executing a set-user-ID or set-group-ID program that resides on a filesystem that was mounted with the .BR MS_NOSUID flag, as described in .BR mount (2).) .\" .\" ============================================================ .\" .SS Miscellaneous .PP When a process's user and group IDs are passed over a UNIX domain socket to a process in a different user namespace (see the description of .B SCM_CREDENTIALS in .BR unix (7)), they are translated into the corresponding values as per the receiving process's user and group ID mappings. .\" .SH CONFORMING TO Namespaces are a Linux-specific feature. .\" .SH NOTES Over the years, there have been a lot of features that have been added to the Linux kernel that have been made available only to privileged users because of their potential to confuse set-user-ID-root applications. In general, it becomes safe to allow the root user in a user namespace to use those features because it is impossible, while in a user namespace, to gain more privilege than the root user of a user namespace has. .\" .\" ============================================================ .\" .SS Availability Use of user namespaces requires a kernel that is configured with the .B CONFIG_USER_NS option. User namespaces require support in a range of subsystems across the kernel. When an unsupported subsystem is configured into the kernel, it is not possible to configure user namespaces support. As at Linux 3.8, most relevant subsystems supported user namespaces, but a number of filesystems did not have the infrastructure needed to map user and group IDs between user namespaces. Linux 3.9 added the required infrastructure support for many of the remaining unsupported filesystems (Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2). Linux 3.11 added support the last of the unsupported major filesystems, .\" commit d6970d4b726cea6d7a9bc4120814f95c09571fc3 XFS. .\" .SH EXAMPLE The program below is designed to allow experimenting with user namespaces, as well as other types of namespaces. It creates namespaces as specified by command-line options and then executes a command inside those namespaces. The comments and .I usage() function inside the program provide a full explanation of the program. The following shell session demonstrates its use. First, we look at the run-time environment: .in +4n .nf $ \fBuname -rs\fP # Need Linux 3.8 or later Linux 3.8.0 $ \fBid -u\fP # Running as unprivileged user 1000 $ \fBid -g\fP 1000 .fi .in Now start a new shell in new user .RI ( \-U ), mount .RI ( \-m ), and PID .RI ( \-p ) namespaces, with user ID .RI ( \-M ) and group ID .RI ( \-G ) 1000 mapped to 0 inside the user namespace: .in +4n .nf $ \fB./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash\fP .fi .in The shell has PID 1, because it is the first process in the new PID namespace: .in +4n .nf bash$ \fBecho $$\fP 1 .fi .in Inside the user namespace, the shell has user and group ID 0, and a full set of permitted and effective capabilities: .in +4n .nf bash$ \fBcat /proc/$$/status | egrep '^[UG]id'\fP Uid: 0 0 0 0 Gid: 0 0 0 0 bash$ \fBcat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'\fP CapInh: 0000000000000000 CapPrm: 0000001fffffffff CapEff: 0000001fffffffff .fi .in Mounting a new .I /proc filesystem and listing all of the processes visible in the new PID namespace shows that the shell can't see any processes outside the PID namespace: .in +4n .nf bash$ \fBmount -t proc proc /proc\fP bash$ \fBps ax\fP PID TTY STAT TIME COMMAND 1 pts/3 S 0:00 bash 22 pts/3 R+ 0:00 ps ax .fi .in .SS Program source \& .nf /* userns_child_exec.c Licensed under GNU General Public License v2 or later Create a child process that executes a shell command in new namespace(s); allow UID and GID mappings to be specified when creating a user namespace. */ #define _GNU_SOURCE #include #include #include #include #include #include #include #include #include #include /* A simple error\-handling function: print an error message based on the value in \(aqerrno\(aq and terminate the calling process */ #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\ } while (0) struct child_args { char **argv; /* Command to be executed by child, with args */ int pipe_fd[2]; /* Pipe used to synchronize parent and child */ }; static int verbose; static void usage(char *pname) { fprintf(stderr, "Usage: %s [options] cmd [arg...]\\n\\n", pname); fprintf(stderr, "Create a child process that executes a shell " "command in a new user namespace,\\n" "and possibly also other new namespace(s).\\n\\n"); fprintf(stderr, "Options can be:\\n\\n"); #define fpe(str) fprintf(stderr, " %s", str); fpe("\-i New IPC namespace\\n"); fpe("\-m New mount namespace\\n"); fpe("\-n New network namespace\\n"); fpe("\-p New PID namespace\\n"); fpe("\-u New UTS namespace\\n"); fpe("\-U New user namespace\\n"); fpe("\-M uid_map Specify UID map for user namespace\\n"); fpe("\-G gid_map Specify GID map for user namespace\\n"); fpe("\-z Map user\(aqs UID and GID to 0 in user namespace\\n"); fpe(" (equivalent to: \-M \(aq0 1\(aq \-G \(aq0 1\(aq)\\n"); fpe("\-v Display verbose messages\\n"); fpe("\\n"); fpe("If \-z, \-M, or \-G is specified, \-U is required.\\n"); fpe("It is not permitted to specify both \-z and either \-M or \-G.\\n"); fpe("\\n"); fpe("Map strings for \-M and \-G consist of records of the form:\\n"); fpe("\\n"); fpe(" ID\-inside\-ns ID\-outside\-ns len\\n"); fpe("\\n"); fpe("A map string can contain multiple records, separated" " by commas;\\n"); fpe("the commas are replaced by newlines before writing" " to map files.\\n"); exit(EXIT_FAILURE); } /* Update the mapping file \(aqmap_file\(aq, with the value provided in \(aqmapping\(aq, a string that defines a UID or GID mapping. A UID or GID mapping consists of one or more newline\-delimited records of the form: ID_inside\-ns ID\-outside\-ns length Requiring the user to supply a string that contains newlines is of course inconvenient for command\-line use. Thus, we permit the use of commas to delimit records in this string, and replace them with newlines before writing the string to the file. */ static void update_map(char *mapping, char *map_file) { int fd, j; size_t map_len; /* Length of \(aqmapping\(aq */ /* Replace commas in mapping string with newlines */ map_len = strlen(mapping); for (j = 0; j < map_len; j++) if (mapping[j] == \(aq,\(aq) mapping[j] = \(aq\\n\(aq; fd = open(map_file, O_RDWR); if (fd == \-1) { fprintf(stderr, "ERROR: open %s: %s\\n", map_file, strerror(errno)); exit(EXIT_FAILURE); } if (write(fd, mapping, map_len) != map_len) { fprintf(stderr, "ERROR: write %s: %s\\n", map_file, strerror(errno)); exit(EXIT_FAILURE); } close(fd); } static int /* Start function for cloned child */ childFunc(void *arg) { struct child_args *args = (struct child_args *) arg; char ch; /* Wait until the parent has updated the UID and GID mappings. See the comment in main(). We wait for end of file on a pipe that will be closed by the parent process once it has updated the mappings. */ close(args\->pipe_fd[1]); /* Close our descriptor for the write end of the pipe so that we see EOF when parent closes its descriptor */ if (read(args\->pipe_fd[0], &ch, 1) != 0) { fprintf(stderr, "Failure in child: read from pipe returned != 0\\n"); exit(EXIT_FAILURE); } /* Execute a shell command */ printf("About to exec %s\\n", args\->argv[0]); execvp(args\->argv[0], args\->argv); errExit("execvp"); } #define STACK_SIZE (1024 * 1024) static char child_stack[STACK_SIZE]; /* Space for child\(aqs stack */ int main(int argc, char *argv[]) { int flags, opt, map_zero; pid_t child_pid; struct child_args args; char *uid_map, *gid_map; const int MAP_BUF_SIZE = 100; char map_buf[MAP_BUF_SIZE]; char map_path[PATH_MAX]; /* Parse command\-line options. The initial \(aq+\(aq character in the final getopt() argument prevents GNU\-style permutation of command\-line options. That\(aqs useful, since sometimes the \(aqcommand\(aq to be executed by this program itself has command\-line options. We don\(aqt want getopt() to treat those as options to this program. */ flags = 0; verbose = 0; gid_map = NULL; uid_map = NULL; map_zero = 0; while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != \-1) { switch (opt) { case \(aqi\(aq: flags |= CLONE_NEWIPC; break; case \(aqm\(aq: flags |= CLONE_NEWNS; break; case \(aqn\(aq: flags |= CLONE_NEWNET; break; case \(aqp\(aq: flags |= CLONE_NEWPID; break; case \(aqu\(aq: flags |= CLONE_NEWUTS; break; case \(aqv\(aq: verbose = 1; break; case \(aqz\(aq: map_zero = 1; break; case \(aqM\(aq: uid_map = optarg; break; case \(aqG\(aq: gid_map = optarg; break; case \(aqU\(aq: flags |= CLONE_NEWUSER; break; default: usage(argv[0]); } } /* \-M or \-G without \-U is nonsensical */ if (((uid_map != NULL || gid_map != NULL || map_zero) && !(flags & CLONE_NEWUSER)) || (map_zero && (uid_map != NULL || gid_map != NULL))) usage(argv[0]); args.argv = &argv[optind]; /* We use a pipe to synchronize the parent and child, in order to ensure that the parent sets the UID and GID maps before the child calls execve(). This ensures that the child maintains its capabilities during the execve() in the common case where we want to map the child\(aqs effective user ID to 0 in the new user namespace. Without this synchronization, the child would lose its capabilities if it performed an execve() with nonzero user IDs (see the capabilities(7) man page for details of the transformation of a process\(aqs capabilities during execve()). */ if (pipe(args.pipe_fd) == \-1) errExit("pipe"); /* Create the child in new namespace(s) */ child_pid = clone(childFunc, child_stack + STACK_SIZE, flags | SIGCHLD, &args); if (child_pid == \-1) errExit("clone"); /* Parent falls through to here */ if (verbose) printf("%s: PID of child created by clone() is %ld\\n", argv[0], (long) child_pid); /* Update the UID and GID maps in the child */ if (uid_map != NULL || map_zero) { snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map", (long) child_pid); if (map_zero) { snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid()); uid_map = map_buf; } update_map(uid_map, map_path); } if (gid_map != NULL || map_zero) { snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map", (long) child_pid); if (map_zero) { snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid()); gid_map = map_buf; } update_map(gid_map, map_path); } /* Close the write end of the pipe, to signal to the child that we have updated the UID and GID maps */ close(args.pipe_fd[1]); if (waitpid(child_pid, NULL, 0) == \-1) /* Wait for child */ errExit("waitpid"); if (verbose) printf("%s: terminating\\n", argv[0]); exit(EXIT_SUCCESS); } .fi .SH SEE ALSO .BR newgidmap (1), \" From the shadow package .BR newuidmap (1), \" From the shadow package .BR clone (2), .BR setns (2), .BR unshare (2), .BR proc (5), .BR subgid (5), \" From the shadow package .BR subuid (5), \" From the shadow package .BR credentials (7), .BR capabilities (7), .BR namespaces (7), .BR pid_namespaces (7) .sp The kernel source file .IR Documentation/namespaces/resource-control.txt . .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.