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

[Thomas Huth <thuth@redhat.com>]

On 08/07/2018 02:58 PM, Daniel P. Berrangé wrote:
> On Tue, Aug 07, 2018 at 01:52:24PM +0100, Dr. David Alan Gilbert wrote:
>> * Peter Maydell (peter.maydell@linaro.org) 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
> 
> IIUC, this changeset was a security fix for CVE-2018-11806.
> 
> Given that the fix was flawed and allowed guest to crash the host
> with a new buffer overrun, it seems we need to get a new CVE allocated
> too.

But 864036e251f54c9 was never part of an official QEMU release, was it?
Or did it go into a stable release already? If not, I think you simply
need both patches to fix the CVE instead.

 Thomas