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>
diff --git a/fs/namespace.c b/fs/namespace.c
index ffa3843..8174c8a 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1136,8 +1136,15 @@
{
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 e764f24..5b3182c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1305,6 +1305,8 @@
#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;
