* Peter Maydell ([email protected]) 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 <[email protected]>
> ---
> 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.
Dave
>
> --
> 2.17.1
>
>
--
Dr. David Alan Gilbert / [email protected] / Manchester, UK
