From 42e9b08a7a6151b048540cb312b100a0da5cc497 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 2 Apr 2017 17:10:08 -0700 Subject: [PATCH] vfs: don't do RCU lookup of empty pathnames commit c0eb027e5aef70b71e5a38ee3e264dc0b497f343 upstream. 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 Reported-by: Vegard Nossum Acked-by: Al Viro Signed-off-by: Linus Torvalds Cc: Eric Biggers Signed-off-by: Greg Kroah-Hartman --- fs/namei.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/namei.c b/fs/namei.c index 3f96ae087488..844da20232b9 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -2000,6 +2000,9 @@ 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;