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xwayland: Relax the ownership requirements of /tmp/.X11-unix

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The `ensure_x11_unix_perms` function tries to detect systems on which
/tmp/.X11-unix is owned by neither root nor ourselves because in that
case the owner can take over the socket we create (symlink races are
fixed in linux 800179c9b8a1e796e441674776d11cd4c05d61d7). This should
not be possible in the first place and systems should come with some way
to ensure that's the case (systemd-tmpfiles, polyinstantiationm …). That
check however only works if we see the root user namespace which might
not be the case when running in e.g. toolbx.

This change relaxes the requirements such that in the root user
namespace we detect and abort if a vulnerable system is detected but
unconditionally run in toolbx.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2261>
This commit is contained in:
Sebastian Wick 2022-01-28 15:49:11 +01:00 committed by Marge Bot
parent 6427b27602
commit abadb29132
1 changed files with 26 additions and 6 deletions
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32
src/wayland/meta-xwayland.c
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@ -60,6 +60,7 @@
#define XWAYLAND_LISTENFD "-listen" #define XWAYLAND_LISTENFD "-listen"
#endif #endif
#define TMP_UNIX_DIR "/tmp"
#define X11_TMP_UNIX_DIR "/tmp/.X11-unix" #define X11_TMP_UNIX_DIR "/tmp/.X11-unix"
#define X11_TMP_UNIX_PATH "/tmp/.X11-unix/X" #define X11_TMP_UNIX_PATH "/tmp/.X11-unix/X"
@ -496,9 +497,18 @@ meta_xwayland_override_display_number (int number)
static gboolean static gboolean
ensure_x11_unix_perms (GError **error) ensure_x11_unix_perms (GError **error)
{ {
struct stat buf; /* Try to detect systems on which /tmp/.X11-unix is owned by neither root nor
* ourselves because in that case the owner can take over the socket we create
* (symlink races are fixed in linux 800179c9b8a1). This should not be
* possible in the first place and systems should come with some way to ensure
* that's the case (systemd-tmpfiles, polyinstantiation …).
*
* That check however only works if we see the root user namespace which might
* not be the case when running in e.g. toolbx (root and other user are all
* mapped to overflowuid). */
struct stat x11_tmp, tmp;
if (lstat (X11_TMP_UNIX_DIR, &buf) != 0) if (lstat (X11_TMP_UNIX_DIR, &x11_tmp) != 0)
{ {
g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno), g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno),
"Failed to check permissions on directory \"%s\": %s", "Failed to check permissions on directory \"%s\": %s",
@ -506,8 +516,18 @@ ensure_x11_unix_perms (GError **error)
return FALSE; return FALSE;
} }
/* If the directory already exists, it should belong to root or ourselves ... */ if (lstat (TMP_UNIX_DIR, &tmp) != 0)
if (buf.st_uid != 0 && buf.st_uid != getuid ()) {
g_set_error (error, G_IO_ERROR, g_io_error_from_errno (errno),
"Failed to check permissions on directory \"%s\": %s",
TMP_UNIX_DIR, g_strerror (errno));
return FALSE;
}
/* If the directory already exists, it should belong to the same
* user as /tmp or belong to ourselves ...
* (if /tmp is not owned by root or ourselves we're in deep trouble) */
if (x11_tmp.st_uid != tmp.st_uid && x11_tmp.st_uid != getuid ())
{ {
g_set_error (error, G_IO_ERROR, G_IO_ERROR_PERMISSION_DENIED, g_set_error (error, G_IO_ERROR, G_IO_ERROR_PERMISSION_DENIED,
"Wrong ownership for directory \"%s\"", "Wrong ownership for directory \"%s\"",
@ -516,7 +536,7 @@ ensure_x11_unix_perms (GError **error)
} }
/* ... be writable ... */ /* ... be writable ... */
if ((buf.st_mode & 0022) != 0022) if ((x11_tmp.st_mode & 0022) != 0022)
{ {
g_set_error (error, G_IO_ERROR, G_IO_ERROR_PERMISSION_DENIED, g_set_error (error, G_IO_ERROR, G_IO_ERROR_PERMISSION_DENIED,
"Directory \"%s\" is not writable", "Directory \"%s\" is not writable",
@ -525,7 +545,7 @@ ensure_x11_unix_perms (GError **error)
} }
/* ... and have the sticky bit set */ /* ... and have the sticky bit set */
if ((buf.st_mode & 01000) != 01000) if ((x11_tmp.st_mode & 01000) != 01000)
{ {
g_set_error (error, G_IO_ERROR, G_IO_ERROR_PERMISSION_DENIED, g_set_error (error, G_IO_ERROR, G_IO_ERROR_PERMISSION_DENIED,
"Directory \"%s\" is missing the sticky bit", "Directory \"%s\" is missing the sticky bit",