Re: [Xen-devel] [PATCH] Re: xen-4.3.1:hvm.c: 2 * possible bad if tests ?

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/11/13 15:03, Tim Deegan wrote:
> At 11:54 +0000 on 21 Nov (1385031246), Andrew Cooper wrote:
>> On 21/11/13 11:45, David Binderman wrote:
>>> Hello there,
>>>
>>> I just ran the source code of xen-4.3.1 through the static analyser 
>>> "cppcheck".
>>>
>>> It said
>>>
>>> 1.
>>>
>>> [hvm.c:2190]: (style) Expression '(X & 0xc00) != 0x6' is always true.
>>>
>>> Source code is
>>>
>>>             if ( ((desc.b & (6u<<9)) != 6) && (dpl != rpl) )
>>>                 goto unmap_and_fail;
>>>
>>> You might be better off with
>>>
>>>             if ( ((desc.b & (6u<<9))) && (dpl != rpl) )
>>>                 goto unmap_and_fail;
>>>
>>> 2.
>>>
>>> [hvm.c:2210]: (style) Expression '(X & 0xc00) != 0x6' is always true.
>>>
>>> Source code is
>>>
>>>             if ( ((desc.b & (6u<<9)) != 6) && ((dpl < cpl) || (dpl < rpl)) )
>>>                 goto unmap_and_fail;
>> These have both been flagged up by our Coverity scanning, but I haven't
>> had enough time to pour over the manuals workout out the correct
>> expression should be.
>>
>> The prevailing style for all other checks in this area is "(X & (6u<<9))
>> != (6u<<9)" , which is rather different to the result you came up with.
>>
>> As this is the security checks for segment selectors in the emulation
>> code, leaving it in its current "too many operations are failed" is
>> safer than being uncertain with the fix and introducing a vulnerability.
> Looking at the manual and the comment, I think the right change is:
>
> x86/hvm: fix test for non-conforming segments.
>
> Reported-by: David Binderman <dcb314@xxxxxxxxxxx>
> Signed-off-by: Tim Deegan <tim@xxxxxxx>
>
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -2278,7 +2278,7 @@ static int hvm_load_segment_selector(
>              if ( !(desc.b & (1u<<11)) )
>                  goto unmap_and_fail;
>              /* Non-conforming segment: check DPL against RPL. */
> -            if ( ((desc.b & (6u<<9)) != 6) && (dpl != rpl) )
> +            if ( !(desc.b & (1u<<10)) && (dpl != rpl) )
>                  goto unmap_and_fail;
>              break;
>          case x86_seg_ss:
>

There is another example higher in the switch statement for the code
segment selector.

Also, the commit should probably have CID 1055180 referenced ?

~Andrew

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