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 (file)
--- 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 e764f24..5b3182c 100644 (file)
--- 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;