On 1/13/20 11:18 AM, Stephen Smalley wrote:
commit 2db154b3ea8e ("vfs: syscall: Add move_mount(2) to move mounts around")
introduced a new move_mount(2) system call and a corresponding new LSM
security_move_mount hook but did not implement this hook for any existing
LSM. This creates a regression for SELinux with respect to consistent
checking of mounts; the existing selinux_mount hook checks mounton
permission to the mount point path. Provide a SELinux hook
implementation for move_mount that applies this same check for
consistency. We may wish to consider defining a new filesystem
move_mount permission and/or a new dir(ectory) move_mount permission
and checking it in this hook in the future.
Fixes: 2db154b3ea8e ("vfs: syscall: Add move_mount(2) to move mounts around")
Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
NB I cc'd lsm list on this patch just as a heads-up/reminder that this
hook hasn't been implemented for any security modules AFAICT, not just
SELinux. I see that there was some discussion of this in the past with
a trivial patch proposed by Tetsuo to just disable the syscall when
TOMOYO or AppArmor is enabled, but no action seems to have been taken,
https://lore.kernel.org/linux-security-module/5802b8b1-f734-1670-f83b-465eda133936@xxxxxxxxxxxxxxxxxxx/
https://lore.kernel.org/linux-security-module/1565365478-6550-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx/
The move_mount syscall does check may_mount() and hence requires
CAP_SYS_ADMIN for the user namespace associated with the mount
namespace, so both SELinux and AppArmor would at least restrict the use
of this syscall to processes allowed CAP_SYS_ADMIN by policy, but TOMOYO
doesn't implement the capable hook either so move_mount is entirely
unrestricted by it at present. Looks like Smack doesn't implement any
mount checking so it doesn't care about move_mount (especially since it
requires CAP_SYS_ADMIN already).
---
security/selinux/hooks.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 0606e107fca3..244874b103ff 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2766,6 +2766,19 @@ static int selinux_mount(const char *dev_name,
return path_has_perm(cred, path, FILE__MOUNTON);
}
+static int selinux_move_mount(const struct path *from_path,
+ const struct path *to_path)
+{
+ const struct cred *cred = current_cred();
+
+ /*
+ * TBD: Check new FILESYSTEM__MOVE_MOUNT permission to
+ * from_path->dentry->s_sb and/or new DIR__MOVE_MOUNT
+ * permission to from_path?
+ */
+ return path_has_perm(cred, to_path, FILE__MOUNTON);
+}
+
static int selinux_umount(struct vfsmount *mnt, int flags)
{
const struct cred *cred = current_cred();
@@ -6943,6 +6956,8 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(sb_set_mnt_opts, selinux_set_mnt_opts),
LSM_HOOK_INIT(sb_clone_mnt_opts, selinux_sb_clone_mnt_opts),
+ LSM_HOOK_INIT(move_mount, selinux_move_mount),
+
LSM_HOOK_INIT(dentry_init_security, selinux_dentry_init_security),
LSM_HOOK_INIT(dentry_create_files_as, selinux_dentry_create_files_as),
