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vfs: add NOFOLLOW flag to umount(2)

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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>
This commit is contained in:
Miklos Szeredi 2010-02-10 12:15:53 +01:00 committed by Al Viro
parent 440b3c6c16
commit db1f05bb85
2 changed files with 10 additions and 1 deletions
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9
fs/namespace.c
View File

@ -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;

2
include/linux/fs.h
View File

@ -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;