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SubjectRe: recvmmsg/sendmmsg result types inconsistent, integer overflows?
Rich,

On Wed, Jun 11, 2014 at 5:15 PM, Rich Felker <dalias@libc.org> wrote:
> On Tue, Jun 10, 2014 at 10:50:08PM -0700, Eric Dumazet wrote:
>> On Wed, 2014-06-11 at 07:24 +0200, Mike Galbraith wrote:
>> > (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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>> > > Please read the FAQ at http://www.tux.org/lkml/
>> >
>>
>>
>> See commit 8acfe468b0384e834a303f08ebc4953d72fb690a
>> ("net: Limit socket I/O iovec total length to INT_MAX.")
>>
>> (or grep for verify_iovec() )
>
> Thanks; that addresses my concern about safety. There's still the ugly
> API inconsistency which it seems too late to fix. Michael, perhaps
> this should at least be documented in the man pages for sendmmsg and
> recvmmsg since it's certain to be confusing to anyone familiar with
> the sendmsg and recvmsg API, but not with kernel internals, who's
> trying to use these functions...

Care to send me a patch?

Thanks,

Michael


--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/


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