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Re: [Xen-devel] [PATCH 1/2] tools/libxc: Fix issues with libxc and Xen having different featureset lengths
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 30 Nov 2018 13:10:37 +0000
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- Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
- Delivery-date: Fri, 30 Nov 2018 13:11:31 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 30/11/2018 09:18, Jan Beulich wrote:
>>>> On 29.11.18 at 20:20, <andrew.cooper3@xxxxxxxxxx> wrote:
>> In almost all cases, Xen and libxc will agree on the featureset length,
>> because they are built from the same source.
>>
>> However, there are circumstances (e.g. security hotfixes) where the
>> featureset
>> gets longer and dom0 will, after installing updates, be running with an old
>> Xen but new libxc. Despite writing the code with this scenario in mind,
>> there
>> were some bugs.
>>
>> First, xen-cpuid's get_featureset() erroneously allocates a buffer based on
>> Xen's featureset length, but records libxc's length, which is longer.
> "... which may be longer", seeing that nr_features gets initialized from
> xc_get_cpu_featureset_size()'s return value, and its subsequent
> updating (through xc_get_cpu_featureset()) is only done in certain
> cases.
Ah yes - very true. I'll also tweak the following sentence to start
with "In this situation, the hypercall bounce ..."
>
>> The hypercall bounce buffer code reads/writes the recorded length, which is
>> beyond the end of the allocated object, and a later free() encounters corrupt
>> heap metadata. Fix this by recording the same length that we allocate.
>>
>> Secondly, get_cpuid_domain_info() has a related bug when the passed-in
>> featureset is a different length to libxc's.
>>
>> A large amount of the libxc cpuid functionality depends on info->featureset
>> being as long as expected, and it is allocated appropriately. However, in
>> the
>> case that a shorter external featureset is passed in, the logic to check for
>> trailing nonzero bits may read off the end of it. Rework the logic to use
>> the
>> correct upper bound.
>>
>> In addition, leave a comment next to the fields in struct cpuid_domain_info
>> explaining the relationship between the various lengths, and how to cope with
>> different lengths.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Thanks,
~Andrew
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