netdev - Re: recvmmsg/sendmmsg result types inconsistent, integer overflows?

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Date:	Wed, 11 Jun 2014 07:24:21 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Rich Felker <dalias@...c.org>
Cc:	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: recvmmsg/sendmmsg result types inconsistent, integer overflows?

(CCs network wizard hangout)

On Wed, 2014-06-11 at 00:12 -0400, Rich Felker wrote: 
> While looking to add support for the recvmmsg and sendmmsg syscalls in
> musl libc, I ran into some disturbing findings on the kernel side. In
> the struct mmsghdr, the field where the result for each message is
> stored has type int, which is inconsistent with the return type
> ssize_t of recvmsg/sendmsg. So I tried to track down what happens when
> the result is or would be larger than 2GB, and quickly found an
> explanation for why the type in the structure was defined wrong:
> internally, the kernel uses int as the return type for revcmsg and
> sendmsg. Oops.
> 
> A bit more RTFS'ing brought me to tcp_sendmsg in net/ipv4/tcp.c (I
> figured let's look at a stream-based protocol, since datagrams can
> likely never be that big for any existing protocol), and as far as I
> can tell, it's haphazardly mixing int and size_t with no checks for
> overflows. I looked for anywhere the kernel might try to verify before
> starting that the sum of the lengths of all the iovec components
> doesn't overflow INT_MAX or even SIZE_MAX, but didn't find any such
> checks.
> 
> Is there some magic that makes this all safe, or is this a big mess of
> possibly-security-relevant bugs?
> 
> Rich
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