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

[Michael Kerrisk]

[adding developers of the two syscalls to CC; maybe they have some insights.]

On Wed, Jun 11, 2014 at 6:12 AM, Rich Felker <dalias@xxxxxxxx> 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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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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