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Re: [Xen-devel] [PATCH] x86/msi: fix loop termination condition in pci_msi_conf_write_intercept()
- To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 2 Jul 2019 11:32:49 +0100
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- Cc: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 02 Jul 2019 10:32:56 +0000
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On 02/07/2019 11:31, Paul Durrant wrote:
>> -----Original Message-----
>> From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
>> Sent: 02 July 2019 11:29
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Cc: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Jan
>> Beulich <jbeulich@xxxxxxxx>;
>> Roger Pau Monne <roger.pau@xxxxxxxxxx>
>> Subject: Re: [Xen-devel] [PATCH] x86/msi: fix loop termination condition in
>> pci_msi_conf_write_intercept()
>>
>> On 02/07/2019 10:47, Andrew Cooper wrote:
>>> On 02/07/2019 10:34, Paul Durrant wrote:
>>>> The for loop that deals with MSI masking is coded as follows:
>>>>
>>>> for ( pos = 0; pos < entry->msi.nvec; ++pos, ++entry )
>>>>
>>>> Thus the loop termination condition is dereferencing a struct pointer that
>>>> is being incremented by the loop. However, it is clear from following code
>>>> paths in msi_capability_init() that this is unsafe as for instance, in the
>>>> case of nvec == 1, entry will point at a single struct msi_desc allocation
>>>> and thus the loop will walk beyond the bounds of the allocation before
>>>> dereferencing the memory to determine whether the loop should terminate.
>>> More specifically, only entry[0].msi.nvec is correct. All subsequent
>>> nvec fields are 0 in a block of entries.
>>>
>>>> Also, because the body of the loop writes via the entry pointer, this can
>>>> then lead to heap memory corruption, or indeed corruption of anything in
>>>> the direct map.
>>>>
>>>> This patch simply initializes a stack variable to the value of
>>>> entry->msi.nvec before starting the loop and then uses that in the
>>>> termination condition instead.
>> There is actually a second bug here which is being fixed. How about
>> this for the commit message?
>>
> Apart from exchange/outlook terminally mangling it (as you can probably see
> below... unless it miraculously unmangles this reply), it looks ok to me. I
> assume you are happy to fix on commit?
Yeah - that is horrifically mangled. The actual commit reads sensibly.
I'm happy to fix on commit.
~Andrew
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