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

[Tim Deegan <tim@xxxxxxx>]

At 11:50 +0000 on 22 Nov (1385117445), Jan Beulich wrote:
> >>> On 21.11.13 at 19:56, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 21/11/13 15:32, Tim Deegan wrote:
> >> At 16:13 +0100 on 21 Nov (1385046788), Tim Deegan wrote:
> >>> At 15:07 +0000 on 21 Nov (1385042827), Andrew Cooper wrote:
> >>>> 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 ?
> >>> Sure.  here's v2:
> >> ...which was buggy: one path needs to handle data segments too.  v3:
> >>
> >> commit 8f8b746cfdcc11197c91efea2b4414045e846fa3
> >> Author: Tim Deegan <tim@xxxxxxx>
> >> Date:   Thu Nov 21 15:11:39 2013 +0000
> >>
> >>     x86/hvm: fix test for non-conforming segments.
> >>     
> >>     Also Coverity CID 1055180
> >>     
> >>     Reported-by: David Binderman <dcb314@xxxxxxxxxxx>
> >>     Signed-off-by: Tim Deegan <tim@xxxxxxx>
> >>
> >> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> >> index 3b353ec..d64f635 100644
> >> --- 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:
> >> @@ -2298,7 +2298,8 @@ static int hvm_load_segment_selector(
> >>              if ( (desc.b & (5u<<9)) == (4u<<9) )
> >>                  goto unmap_and_fail;
> >>              /* Non-conforming segment: check DPL against RPL and CPL. */
> >> -            if ( ((desc.b & (6u<<9)) != 6) && ((dpl < cpl) || (dpl < 
> >> rpl)) )
> >> +            if ( ((desc.b & (3u<<10)) != (3u<<10))
> >> +                 && ((dpl < cpl) || (dpl < rpl)) )
> >>                  goto unmap_and_fail;
> >>              break;
> >>          }
> > 
> > Can you fix the comment to /* Data or non-conforming segment: check DPL
> > against RPL and CPL. */ to match the new logic?

Yes.

> And ideally use _SEGMENT_* instead of raw numbers...

Eh, OK.  It'll be next week, then, with a followup to convert the
surrounding code too.

Tim.

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