vfs: add NOFOLLOW flag to umount(2)

Add a new UMOUNT_NOFOLLOW flag to umount(2).  This is needed to prevent
symlink attacks in unprivileged unmounts (fuse, samba, ncpfs).

Additionally, return -EINVAL if an unknown flag is used (and specify
an explicitly unused flag: UMOUNT_UNUSED).  This makes it possible for
the caller to determine if a flag is supported or not.

CC: Eugene Teo <eugene@redhat.com>
CC: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 10 insertions, 1 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index ffa3843404e0..8174c8ab5c70 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1136,8 +1136,15 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
 {
         struct path path;
         int retval;
+        int lookup_flags = 0;
 
-        retval = user_path(name, &path);
+        if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
+                return -EINVAL;
+
+        if (!(flags & UMOUNT_NOFOLLOW))
+                lookup_flags |= LOOKUP_FOLLOW;
+
+        retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
         if (retval)
                 goto out;
         retval = -EINVAL;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e764f247d0ab..5b3182c7eb5f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1305,6 +1305,8 @@ extern int send_sigurg(struct fown_struct *fown);
 #define MNT_FORCE       0x00000001      /* Attempt to forcibily umount */
 #define MNT_DETACH      0x00000002      /* Just detach from the tree */
 #define MNT_EXPIRE      0x00000004      /* Mark for expiry */
+#define UMOUNT_NOFOLLOW 0x00000008      /* Don't follow symlink on umount */
+#define UMOUNT_UNUSED   0x80000000      /* Flag guaranteed to be unused */
 
 extern struct list_head super_blocks;
 extern spinlock_t sb_lock;