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

[Rich Felker]

On Thu, Jun 12, 2014 at 08:04:54AM +0200, Michael Kerrisk (man-pages) wrote:
> Rich,
> 
> On Wed, Jun 11, 2014 at 5:15 PM, Rich Felker <dalias@xxxxxxxx> 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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> >> >
> >>
> >>
> >> 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?

I'm not at all familiar with man format, but I could write some
suggested text if that would be ok.

Unrelated (at least not directly) to man pages, one concern I have is
that, if these interfaces move towards standardization, the
inconsistent return type is going to be a point of contention that
could result in an incompatible version of the interfaces being
adopted in the standard. From that standpoint, it might make sense to
do something like documenting them returning socklen_t rather than
int (IIRC these are always the same on Linux). It's not quite logical,
but at least a bit more logical from a non-Linux-centric perspective
than using int.

Rich
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