Re: [PATCH v2 07/13] KVM: x86: Protect MSR-based index computations in fixed_msr_to_seg_unit() from Spectre-v1/L1TF attacks

[Marios Pomonis <pomonis@xxxxxxxxxx>]

Hi Sasha,

These build issues can be fixed by including linux/nospec.h to
arch/x86/kvm/mtrr.c. Below you can find a patch that compiles on both
v4.9.206 and v4.4.206.

Please let me know if you need anything else.

Marios

========
diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
index 0149ac59c273..f223f1315998 100644
--- a/arch/x86/kvm/mtrr.c
+++ b/arch/x86/kvm/mtrr.c
@@ -18,6 +18,7 @@

 #include <linux/kvm_host.h>
 #include <asm/mtrr.h>
+#include <linux/nospec.h>

 #include "cpuid.h"
 #include "mmu.h"
@@ -202,11 +203,15 @@ static bool fixed_msr_to_seg_unit(u32 msr, int
*seg, int *unit)
                break;
        case MSR_MTRRfix16K_80000 ... MSR_MTRRfix16K_A0000:
                *seg = 1;
-               *unit = msr - MSR_MTRRfix16K_80000;
+               *unit = array_index_nospec(
+                       msr - MSR_MTRRfix16K_80000,
+                       MSR_MTRRfix16K_A0000 - MSR_MTRRfix16K_80000 + 1);
                break;
        case MSR_MTRRfix4K_C0000 ... MSR_MTRRfix4K_F8000:
                *seg = 2;
-               *unit = msr - MSR_MTRRfix4K_C0000;
+               *unit = array_index_nospec(
+                       msr - MSR_MTRRfix4K_C0000,
+                       MSR_MTRRfix4K_F8000 - MSR_MTRRfix4K_C0000 + 1);
                break;
        default:
                return false;

On Wed, Dec 25, 2019 at 3:55 PM Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> Hi,
>
> [This is an automated email]
>
> This commit has been processed because it contains a "Fixes:" tag,
> fixing commit: de9aef5e1ad6 ("KVM: MTRR: introduce fixed_mtrr_segment table").
>
> The bot has tested the following trees: v5.4.5, v5.3.18, v4.19.90, v4.14.159, v4.9.206, v4.4.206.
>
> v5.4.5: Build OK!
> v5.3.18: Build OK!
> v4.19.90: Build OK!
> v4.14.159: Build OK!
> v4.9.206: Build failed! Errors:
>     arch/x86/kvm/mtrr.c:205:11: error: implicit declaration of function ‘array_index_nospec’; did you mean ‘array_index_mask_nospec’? [-Werror=implicit-function-declaration]
>
> v4.4.206: Build failed! Errors:
>     arch/x86/kvm/mtrr.c:205:11: error: implicit declaration of function ‘array_index_nospec’; did you mean ‘array_index_mask_nospec’? [-Werror=implicit-function-declaration]
>
>
> NOTE: The patch will not be queued to stable trees until it is upstream.
>
> How should we proceed with this patch?
>
> --
> Thanks,
> Sasha



-- 
Marios Pomonis
Software Engineer, Security
GCP Platform Security
US-KIR-6THC