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vfs: don't do RCU lookup of empty pathnames

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Normal pathname lookup doesn't allow empty pathnames, but using
AT_EMPTY_PATH (with name_to_handle_at() or fstatat(), for example) you
can trigger an empty pathname lookup.

And not only is the RCU lookup in that case entirely unnecessary
(because we'll obviously immediately finalize the end result), it is
actively wrong.

Why? An empth path is a special case that will return the original
'dirfd' dentry - and that dentry may not actually be RCU-free'd,
resulting in a potential use-after-free if we were to initialize the
path lazily under the RCU read lock and depend on complete_walk()
finalizing the dentry.

Found by syzkaller and KASAN.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2017-04-02 17:10:08 -07:00
parent 1bf4b1268e
commit c0eb027e5a
1 changed files with 3 additions and 0 deletions
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3
fs/namei.c
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@ -2145,6 +2145,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
int retval = 0;
const char *s = nd->name->name;
if (!*s)
flags &= ~LOOKUP_RCU;
nd->last_type = LAST_ROOT; /* if there are only slashes... */
nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT;
nd->depth = 0;