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Re: [Qemu-devel] [PATCH for-3.0] slirp: Correct size check in m_inc()

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH for-3.0] slirp: Correct size check in m_inc()
Date: Thu, 9 Aug 2018 12:12:33 +0100
User-agent: Mutt/1.10.1 (2018-07-13)

* Peter Maydell (address@hidden) wrote:
> The data in an mbuf buffer is not necessarily at the start of the
> allocated buffer. (For instance m_adj() allows data to be trimmed
> from the start by just advancing the pointer and reducing the length.)
> This means that the allocated buffer size (m->m_size) and the
> amount of space from the m_data pointer to the end of the
> buffer (M_ROOM(m)) are not necessarily the same.
> Commit 864036e251f54c9 tried to change the m_inc() function from
> taking the new allocated-buffer-size to taking the new room-size,
> but forgot to change the initial "do we already have enough space"
> check. This meant that if we were trying to extend a buffer which
> had a leading gap between the buffer start and the data, we might
> incorrectly decide it didn't need to be extended, and then
> overrun the end of the buffer, causing memory corruption and
> an eventual crash.
> Change the "already big enough?" condition from checking the
> argument against m->m_size to checking against M_ROOM().
> This only makes a difference for the callsite in m_cat();
> the other three callsites all start with a freshly allocated
> mbuf from m_get(), which will have m->m_size == M_ROOM(m).
> Fixes: 864036e251f54c9
> Fixes: https://bugs.launchpad.net/qemu/+bug/1785670
> Signed-off-by: Peter Maydell <address@hidden>
> ---
>  slirp/mbuf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/slirp/mbuf.c b/slirp/mbuf.c
> index 0c189e1a7bf..1b7868355a3 100644
> --- a/slirp/mbuf.c
> +++ b/slirp/mbuf.c
> @@ -154,7 +154,7 @@ m_inc(struct mbuf *m, int size)
>      int datasize;
>      /* some compilers throw up on gotos.  This one we can fake. */
> -    if (m->m_size > size) {
> +    if (M_ROOM(m) > size) {
>          return;
>      }

I'm worried about a side effect of this change.
A few lines below we have:

        datasize = m->m_data - m->m_dat;
        m->m_ext = g_malloc(size + datasize);
        memcpy(m->m_ext, m->m_dat, m->m_size);
        m->m_flags |= M_EXT;

Question: What guarantees there's m_size room for that
memcpy in the new m_ext?

Before this fix, it used to be the case that m_size was
smaller than size, so we knew it fitted; but that's
no longer true.

I don't think I understand the relationship between datasize
and m_size enough to know if anything is sufficient.


> -- 
> 2.17.1
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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