Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 7 Jul 2017 17:23:13 -0700 | Subject | Re: [Patch] mqueue: fix the retry logic for netlink_attachskb() |
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On Fri, Jul 7, 2017 at 11:32 AM, Cong Wang <xiyou.wangcong@gmail.com> wrote: > The retry logic for netlink_attachskb() inside sys_mq_notify() > is suspicious and vulnerable: > > 1) The sock refcnt is already released when retry is needed > 2) The fd is controllable by user-space because we already > release the file refcnt
Hmm. What's different the second (and third.. and..) time around from the first time?
I don't dislike your patch (it looks fine), but avoiding the fdget/fdput in the retry loop doesn't seem to really change anything - it's just as if we'd just react to the original thing a bit later.
> 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?
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.
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.
Linus
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