[PATCH AUTOSEL 5.5 417/542] selinux: fix regression introduced by move_mount(2) syscall

[Sasha Levin]

From: Stephen Smalley <sds@xxxxxxxxxxxxx>

[ Upstream commit 98aa00345de54b8340dc2ddcd87f446d33387b5e ]

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.  In the future we may wish to add a new move_mount
filesystem permission and check as well, but this addresses
the immediate regression.

Fixes: 2db154b3ea8e ("vfs: syscall: Add move_mount(2) to move mounts around")
Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>
Reviewed-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 security/selinux/hooks.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 65641c61ecb94..db44c7eb43213 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2762,6 +2762,14 @@ 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();
+
+	return path_has_perm(cred, to_path, FILE__MOUNTON);
+}
+
 static int selinux_umount(struct vfsmount *mnt, int flags)
 {
 	const struct cred *cred = current_cred();
@@ -6907,6 +6915,8 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(sb_clone_mnt_opts, selinux_sb_clone_mnt_opts),
 	LSM_HOOK_INIT(sb_add_mnt_opt, selinux_add_mnt_opt),
 
+	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),
 
-- 
2.20.1