Re: Crash with SO_REUSEPORT and ef456144da8ef507c8cf504284b6042e9201a05c

From: Eric Dumazet
Date: Tue Jan 19 2016 - 11:15:09 EST

On Tue, 2016-01-19 at 11:57 -0400, Marc Dionne wrote:
> I shared this one with Craig but I thought I'd put it out to a wider audience.
> Trying to run the current kernel mainline on a test system I found
> that any attempt to run many of our executables would crash the
> system. The networking code in all of these opens and listens on
> multiple UDP sockets set with SO_REUSEPORT. We also like to bind the
> first socket before setting SO_REUSEPORT so we can catch some cases
> where the port is actually in use by someone else (for instance a
> previous incarnation of the same service).
> This is easily reproduced with this sequence:
> - create 2 sockets A and B
> - bind socket A to an address
> - set SO_REUSEPORT on socket A
> - set SO_REUSEPORT on socket B
> - bind socket B to the same address as A
> The sk_reuseport_cb structure is only allocated at bind time if
> SO_REUSEPORT is already set, so A doesn't have one. When we bind B, A
> is found as a match that has SO_REUSEPORT and reuseport_add_sock will
> try to use the NULL sk_reuseport_cb structure from A, causing a crash.
> Not sure what the best fix is, but seems like the structure could be
> either allocated (if not already done) when setting SO_REUSEPORT, or
> when we find it to be NULL in reuseport_add_sock (but locking may be
> an issue there). I was able to test that allocating sk_reuseport_cb
> when setting SO_REUSEPORT makes things behave normally again; see
> attached patch. That's surely not a correct/complete fix as B (in the
> scenario above) will have an unnecessary sk_reuseport_cb which will
> trigger a warning and should be dealt with.

Hi Marc

Your patch looks fine to me, please add a "Fixes:" tag in it ?

Fixes: e32ea7e74727 ("soreuseport: fast reuseport UDP socket selection")