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Subject[PATCH v2] vfs: add NOFOLLOW flag to umount(2)
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From: Miklos Szeredi <mszeredi@suse.cz>

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>
---
v2:
- renamed flag to UMOUNT_NOFOLLOW
- added UMOUNT_UNUSED for feature detection

fs/namespace.c | 9 ++++++++-
include/linux/fs.h | 2 ++
2 files changed, 10 insertions(+), 1 deletion(-)

Index: linux-2.6/fs/namespace.c
===================================================================
--- linux-2.6.orig/fs/namespace.c 2010-02-08 12:00:02.000000000 +0100
+++ linux-2.6/fs/namespace.c 2010-02-09 13:38:03.000000000 +0100
@@ -1121,8 +1121,15 @@ SYSCALL_DEFINE2(umount, char __user *, n
{
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;
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h 2010-02-08 12:00:03.000000000 +0100
+++ linux-2.6/include/linux/fs.h 2010-02-09 13:35:47.000000000 +0100
@@ -1305,6 +1305,8 @@ extern int send_sigurg(struct fown_struc
#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;

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