From: Linus Torvalds torvalds@linux-foundation.org
This patch has been added to the 4.1 stable tree. If you have any objections, please let us know.
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[ Upstream commit c0eb027e5aef70b71e5a38ee3e264dc0b497f343 ]
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 Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- fs/namei.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/fs/namei.c b/fs/namei.c index c7a6eabc02a5..0d97235019a9 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1894,6 +1894,9 @@ static int path_init(int dfd, const struct filename *name, unsigned int flags, int retval = 0; const char *s = 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;