Re: [Xen-devel] [PATCH 3/4] AMD/microcode: Avoid use-after-free for the microcode buffer

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/12/2013 15:30, Boris Ostrovsky wrote:
> On 12/27/2013 09:57 AM, Andrew Cooper wrote:
>> It is possible to free the mc_old buffer and the store it for use on
>> in the
>
> I think you don't need *the* store here. And no *on*.

Oops - I refactored my sentence half way through, and missed that on a
reread.

>
>> case of resume.
>>
>> This keeps the old semantics of being able to return an error even
>> after a
>> successful microcode application.
>>
>> Coverity-ID 1146953
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> CC: Keir Fraser <keir@xxxxxxx>
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>> CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
>> ---
>>   xen/arch/x86/microcode_amd.c |   17 +++++++++--------
>>   1 file changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/xen/arch/x86/microcode_amd.c b/xen/arch/x86/microcode_amd.c
>> index a3ceef8..8ea4e63 100644
>> --- a/xen/arch/x86/microcode_amd.c
>> +++ b/xen/arch/x86/microcode_amd.c
>> @@ -275,7 +275,7 @@ static int cpu_request_microcode(int cpu, const
>> void *buf, size_t bufsize)
>>       struct microcode_amd *mc_amd, *mc_old;
>>       size_t offset = bufsize;
>>       size_t last_offset, applied_offset = 0;
>> -    int error = 0;
>> +    int error = 0, save_error = 1;
>>       struct ucode_cpu_info *uci = &per_cpu(ucode_cpu_info, cpu);
>>         /* We should bind the task to the CPU */
>> @@ -338,19 +338,20 @@ static int cpu_request_microcode(int cpu, const
>> void *buf, size_t bufsize)
>>        */
>>       if ( applied_offset )
>>       {
>> -        int ret = get_ucode_from_buffer_amd(mc_amd, buf, bufsize,
>> -                                            &applied_offset);
>> -        if ( ret == 0 )
>> -            xfree(mc_old);
>> -        else
>> -            error = ret;
>> +        save_error = get_ucode_from_buffer_amd(
>> +            mc_amd, buf, bufsize, &applied_offset);
>> +
>> +        if ( save_error )
>> +            error = save_error
>>       }
>>   -    if ( !applied_offset || error )
>> +    if ( save_error )
>>       {
>>           xfree(mc_amd);
>>           uci->mc.mc_amd = mc_old;
>>       }
>> +    else
>> +        xfree(mc_old);
>
> Won't this free mc_old (which is where previous microcode lived) even
> if the new buffer didn't have any valid microcode (i.e. applied_offset
> is zero)?
>
> -boris

save_error starts off as 1, and only gets set to 0 on a successful
get_ucode_from_buffer_amd(), so we will only free mc_old in the case
that we have valid microcde in mc_amd.  All other cases free mc_amd and
revert to using mc_old.

~Andrew

>
>
>>       out:
>>       svm_host_osvw_init();
>


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