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unshare: allow custom uid/gid mappings in userns
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Two new options are added: `--map-user=<uid>` and `--map-group=<gid>`
for custom user and group mappings respectively. These are just
generalizations of the existing `--map-root-user` and
`--map-current-user` options.

As a side effect of this commit, specifying both `--map-root-user` and
`--map-current-user` no longer causes an error. Instead, the last
occurrence takes precedence.

Addresses: util-linux#885
Signed-off-by: Matthew Harm Bekkema <id@mbekkema.name>
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@mat8913
mat8913 committed Apr 12, 2020
1 parent ea4dcdc commit 6e837b5
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Showing 2 changed files with 42 additions and 36 deletions.
12 changes: 12 additions & 0 deletions sys-utils/unshare.1
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Original file line number Diff line number Diff line change
Expand Up @@ -169,6 +169,16 @@ implies creating a new mount namespace since the /proc mount would otherwise
mess up existing programs on the system. The new proc filesystem is explicitly
mounted as private (with MS_PRIVATE|MS_REC).
.TP
.BR \-\-map\-user=\fIuid
Run the program only after the current effective user ID has been mapped to \fIuid\fP.
If this option is specified multiple times, the last occurrence takes precedence.
This option implies \fB\-\-user\fR.
.TP
.BR \-\-map\-group=\fIgid
Run the program only after the current effective group ID has been mapped to \fIgid\fP.
If this option is specified multiple times, the last occurrence takes precedence.
This option implies \fB\-\-setgroups=deny\fR and \fB\-\-user\fR.
.TP
.BR \-r , " \-\-map\-root\-user"
Run the program only after the current effective user and group IDs have been mapped to
the superuser UID and GID in the newly created user namespace. This makes it possible to
Expand All @@ -177,11 +187,13 @@ namespaces (such as configuring interfaces in the network namespace or mounting
the mount namespace) even when run unprivileged. As a mere convenience feature, it does not support
more sophisticated use cases, such as mapping multiple ranges of UIDs and GIDs.
This option implies \fB\-\-setgroups=deny\fR and \fB\-\-user\fR.
This option is equivalent to \fB\-\-map-user=0 \-\-map-group=0\fR.
.TP
.BR \-c , " \-\-map\-current\-user"
Run the program only after the current effective user and group IDs have been mapped to
the same UID and GID in the newly created user namespace. This option implies
\fB\-\-setgroups=deny\fR and \fB\-\-user\fR.
This option is equivalent to \fB\-\-map-user=$(id -ru) \-\-map-group=$(id -rg)\fR.
.TP
.BR "\-\-propagation private" | shared | slave | unchanged
Recursively set the mount propagation flag in the new mount namespace. The default
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66 changes: 30 additions & 36 deletions sys-utils/unshare.c
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Original file line number Diff line number Diff line change
Expand Up @@ -77,12 +77,6 @@ enum {
SETGROUPS_ALLOW = 1,
};

enum {
MAP_USER_NONE,
MAP_USER_ROOT,
MAP_USER_CURRENT,
};

static const char *setgroups_strings[] =
{
[SETGROUPS_DENY] = "deny",
Expand Down Expand Up @@ -288,6 +282,8 @@ static void __attribute__((__noreturn__)) usage(void)
fputs(_(" -T, --time[=<file>] unshare time namespace\n"), out);
fputs(USAGE_SEPARATOR, out);
fputs(_(" -f, --fork fork before launching <program>\n"), out);
fputs(_(" --map-user=<uid> map current user to uid (implies --user)\n"), out);
fputs(_(" --map-group=<gid> map current group to gid (implies --user)\n"), out);
fputs(_(" -r, --map-root-user map current user to root (implies --user)\n"), out);
fputs(_(" -c, --map-current-user map current user to itself (implies --user)\n"), out);
fputs(USAGE_SEPARATOR, out);
Expand Down Expand Up @@ -323,6 +319,8 @@ int main(int argc, char *argv[])
OPT_KEEPCAPS,
OPT_MONOTONIC,
OPT_BOOTTIME,
OPT_MAPUSER,
OPT_MAPGROUP,
};
static const struct option longopts[] = {
{ "help", no_argument, NULL, 'h' },
Expand All @@ -340,6 +338,8 @@ int main(int argc, char *argv[])
{ "fork", no_argument, NULL, 'f' },
{ "kill-child", optional_argument, NULL, OPT_KILLCHILD },
{ "mount-proc", optional_argument, NULL, OPT_MOUNTPROC },
{ "map-user", required_argument, NULL, OPT_MAPUSER },
{ "map-group", required_argument, NULL, OPT_MAPGROUP },
{ "map-root-user", no_argument, NULL, 'r' },
{ "map-current-user", no_argument, NULL, 'c' },
{ "propagation", required_argument, NULL, OPT_PROPAGATION },
Expand All @@ -356,7 +356,9 @@ int main(int argc, char *argv[])

int setgrpcmd = SETGROUPS_NONE;
int unshare_flags = 0;
int c, forkit = 0, mapuser = MAP_USER_NONE;
int c, forkit = 0;
uid_t mapuser = -1;
gid_t mapgroup = -1;
int kill_child_signo = 0; /* 0 means --kill-child was not used */
const char *procmnt = NULL;
const char *newroot = NULL;
Expand Down Expand Up @@ -428,21 +430,23 @@ int main(int argc, char *argv[])
unshare_flags |= CLONE_NEWNS;
procmnt = optarg ? optarg : "/proc";
break;
case OPT_MAPUSER:
unshare_flags |= CLONE_NEWUSER;
mapuser = strtoul_or_err(optarg, _("failed to parse uid"));
break;
case OPT_MAPGROUP:
unshare_flags |= CLONE_NEWUSER;
mapgroup = strtoul_or_err(optarg, _("failed to parse gid"));
break;
case 'r':
if (mapuser == MAP_USER_CURRENT)
errx(EXIT_FAILURE, _("options --map-root-user and "
"--map-current-user are mutually exclusive"));

unshare_flags |= CLONE_NEWUSER;
mapuser = MAP_USER_ROOT;
mapuser = 0;
mapgroup = 0;
break;
case 'c':
if (mapuser == MAP_USER_ROOT)
errx(EXIT_FAILURE, _("options --map-root-user and "
"--map-current-user are mutually exclusive"));

unshare_flags |= CLONE_NEWUSER;
mapuser = MAP_USER_CURRENT;
mapuser = real_euid;
mapgroup = real_egid;
break;
case OPT_SETGROUPS:
setgrpcmd = setgroups_str2id(optarg);
Expand Down Expand Up @@ -561,33 +565,23 @@ int main(int argc, char *argv[])
if (kill_child_signo != 0 && prctl(PR_SET_PDEATHSIG, kill_child_signo) < 0)
err(EXIT_FAILURE, "prctl failed");

if (mapuser != (uid_t) -1)
map_id(_PATH_PROC_UIDMAP, mapuser, real_euid);

/* Since Linux 3.19 unprivileged writing of /proc/self/gid_map
* has been disabled unless /proc/self/setgroups is written
* first to permanently disable the ability to call setgroups
* in that user namespace. */
switch (mapuser) {
case MAP_USER_ROOT:
if (mapgroup != (gid_t) -1) {
if (setgrpcmd == SETGROUPS_ALLOW)
errx(EXIT_FAILURE, _("options --setgroups=allow and "
"--map-root-user are mutually exclusive"));

"--map-group are mutually exclusive"));
setgroups_control(SETGROUPS_DENY);
map_id(_PATH_PROC_UIDMAP, 0, real_euid);
map_id(_PATH_PROC_GIDMAP, 0, real_egid);
break;
case MAP_USER_CURRENT:
if (setgrpcmd == SETGROUPS_ALLOW)
errx(EXIT_FAILURE, _("options --setgroups=allow and "
"--map-current-user are mutually exclusive"));
map_id(_PATH_PROC_GIDMAP, mapgroup, real_egid);
}

setgroups_control(SETGROUPS_DENY);
map_id(_PATH_PROC_UIDMAP, real_euid, real_euid);
map_id(_PATH_PROC_GIDMAP, real_egid, real_egid);
break;
case MAP_USER_NONE:
if (setgrpcmd != SETGROUPS_NONE)
setgroups_control(setgrpcmd);
}
if (setgrpcmd != SETGROUPS_NONE)
setgroups_control(setgrpcmd);

if ((unshare_flags & CLONE_NEWNS) && propagation)
set_propagation(propagation);
Expand Down

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