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

From: Linus Torvalds
Date: Sat Jul 08 2017 - 14:55:25 EST

On Sat, Jul 8, 2017 at 11:04 AM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>> 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?

It was just the combination of that nasty code, your patch, and the
explanation that confused me.

Reading the patch, I actually thought that one of the things you fixed
was moving the "fdput()" later, to after the netlink_attachskb().

And I thought you did that because netlink_attachskb() would need the
file to be still around keeping a reference count to the socket, and
without it the socket could have been dropped in the meantime.

But reading the code more closely, I notice that
netlink_getsockbyfilp() gets a reference to the sock, and it's that
netlink_attachskb() will drop that reference on error or retry.

So the fdput() makes sense after netlink_getsockbyfilp(), but that's
also why the retry code currently includes repeating the fdget()...
And the error handling for the fdget is that then triggers the real

So the reason you moved the fdput() later wasn't to protect the socket
reference, it was just because of how the whole retry loop needs to
have the file descriptor just to get a new reference to the socket.

That's why I thought you fixed a bug even in the first iteration, but
it turns out that was just me making assumptions based on mis-reading
the patch without looking at all the context and the logic of the
called functions.

Now that I have checked deeper, I realize that your patch description
was actually correct about this only being a retry problem - the first
time around the reference count ends up moving correctly from file to
socket, but then when it repeats and 'sock' may contain a stale
pointer, we may end up doing the wrong thing when the fdget fails.

Honestly, now I feel like either patch is fine, and your original
commit message is fine too - but I just hate that code.

And making it use some nice helper function to clean it up looks
painful too, because the error handling is so odd (ie
mq_netlink_attachskb() will free the skb on error, while the other
error cases won't, so you'd have to have some special handling for the
different errors that can happen).

Honestly, this code is nasty, and right now my feeling is that it
would be good to have a minimal patch that also backports cleanly.
Maybe somebody can clean it up later, but that's a separate windmill
to rail against.

And due to the recent compat cleanups by Al, your bigger patch does
not apply cleanly to current git - but the smaller patch to just
setting 'sock' to NULL before that 'goto retry' should apply cleanly
to all versions of this code.

So purely because of that reason, I think I'd prefer to see that
smaller patch instead. Would you mind re-sending the thing?

Sorry about the whole confusion.