Re: [Patch] mqueue: fix the retry logic for netlink_attachskb()

From: Cong Wang
Date: Sat Jul 08 2017 - 14:05:10 EST

On Fri, Jul 7, 2017 at 5:23 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Jul 7, 2017 at 11:32 AM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>> so we when retry and the fd has been closed during this small
>> window, we end up calling netlink_detachskb() on the error path
>> which releases the sock again and could lead to a use-after-free.
> So this seems to be a real problem: "sock" is not NULL'ed out in that
> if (!f.file) {
> error case (or alternatively, in the retry case). Plus, since we did
> the "fput()" early, "sock" may be gone by the time we do the
> netlink_attachskb() even when it's all successful.
> But I don't think this is really so much about the retrying - the
> "sock may be gone" case seems to be true even the first time around,
> and even if we never retry at all.
> Am I reading this correctly?

Yes you are correct.

> Basically, I think the patch is fine, but the explanation seems a bit
> misleading. This isn't really about the re-trying: that would be fine
> if we just cleaned up sock properly.
> Can you confirm that? I don't know where the original report is.

Yes of course, setting 'sock' to NULL before 'goto retry' is sufficient
to fix it, that is in fact my initial thought. And I realized retry'ing
fdget() can't help anything in this situation but increases the
attack vector, so I decided to get rid of it from the retry loop
instead of just NULL'ing 'sock'.

Or do you prefer the simpler fix? Or should I just resend it with
a improved changelog?

BTW, the original report is here:!topic/syzkaller/QsmbsGoYPzA

> And that code is ancient, so we should do a "cc: stable" there too,
> and backport it basically forever. I think most of the code in this
> area predates the git tree, although Al Viro actually touched some
> things around here very recently to make the compat case cleaner.

Yeah, sorry about forgetting it.