Re: [PATCH 3.13 106/162] KVM: x86: Fix far-jump to non-canonical check

[Nadav Amit]

Vinson Lee <vlee@xxxxxxxxxxxxxxxx> wrote:

> On Thu, Nov 6, 2014 at 2:36 PM, Kamal Mostafa <kamal@xxxxxxxxxxxxx> wrote:
>> 3.13.11.11 -stable review patch.  If anyone has any objections, please let me know.
>> 
>> ------------------
>> 
>> From: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
>> 
>> commit 7e46dddd6f6cd5dbf3c7bd04a7e75d19475ac9f2 upstream.
>> 
>> Commit d1442d85cc30 ("KVM: x86: Handle errors when RIP is set during far
>> jumps") introduced a bug that caused the fix to be incomplete.  Due to
>> incorrect evaluation, far jump to segment with L bit cleared (i.e., 32-bit
>> segment) and RIP with any of the high bits set (i.e, RIP[63:32] != 0) set may
>> not trigger #GP.  As we know, this imposes a security problem.
>> 
>> In addition, the condition for two warnings was incorrect.
>> 
>> Fixes: d1442d85cc30ea75f7d399474ca738e0bc96f715
>> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>> Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
>> [Add #ifdef CONFIG_X86_64 to avoid complaints of undefined behavior. - Paolo]
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> [ kamal: backport to 3.13-stable: omitted WARN_ON fixes (not in 3.13) ]
>> Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
>> ---
>> arch/x86/kvm/emulate.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index a440bea..4ae37e7 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -581,12 +581,14 @@ static inline int assign_eip_far(struct x86_emulate_ctxt *ctxt, ulong dst,
>>        case 4:
>>                ctxt->_eip = (u32)dst;
>>                break;
>> +#ifdef CONFIG_X86_64
>>        case 8:
>>                if ((cs_l && is_noncanonical_address(dst)) ||
>> -                   (!cs_l && (dst & ~(u32)-1)))
>> +                   (!cs_l && (dst >> 32) != 0))
>>                        return emulate_gp(ctxt, 0);
>>                ctxt->_eip = dst;
>>                break;
>> +#endif
>>        default:
>>                WARN(1, "unsupported eip assignment size\n");
>>        }
>> --
>> 1.9.1
>> 
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> 
> 
> Hi.
> 
> Should the WARN_ON fixes have been included as well in 3.13.11.11?
> 
> WARN_ON hunks were added with the backport of "KVM: x86: Handle errors
> when RIP is set during far jumps" in 3.13.11.11 commit
> b8ba339d86fb627d54fea929492114d45f6835c2.

If it was added, it should be fixed. The WARN_ON mistakenly has
double-negation.

Nadav


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