* [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation
@ 2020-01-21 16:11 Paolo Bonzini
2020-01-21 17:24 ` Ben Gardon
0 siblings, 1 reply; 5+ messages in thread
From: Paolo Bonzini @ 2020-01-21 16:11 UTC (permalink / raw)
To: linux-kernel, kvm; +Cc: Sean Christopherson, bgardon
The SPTE_MMIO_MASK overlaps with the bits used to track MMIO
generation number. A high enough generation number would overwrite the
SPTE_SPECIAL_MASK region and cause the MMIO SPTE to be misinterpreted;
likewise, setting bits 52 and 53 would also cause an incorrect generation
number to be read from the PTE.
Fixes: 6eeb4ef049e7 ("KVM: x86: assign two bits to track SPTE kinds")
Reported-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/mmu/mmu.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 57e4dbddba72..e34ca43d9166 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -418,22 +418,25 @@ static inline bool is_access_track_spte(u64 spte)
* requires a full MMU zap). The flag is instead explicitly queried when
* checking for MMIO spte cache hits.
*/
-#define MMIO_SPTE_GEN_MASK GENMASK_ULL(18, 0)
+#define MMIO_SPTE_GEN_MASK GENMASK_ULL(17, 0)
#define MMIO_SPTE_GEN_LOW_START 3
#define MMIO_SPTE_GEN_LOW_END 11
#define MMIO_SPTE_GEN_LOW_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
MMIO_SPTE_GEN_LOW_START)
-#define MMIO_SPTE_GEN_HIGH_START 52
-#define MMIO_SPTE_GEN_HIGH_END 61
+/* Leave room for SPTE_SPECIAL_MASK. */
+#define MMIO_SPTE_GEN_HIGH_START 54
+#define MMIO_SPTE_GEN_HIGH_END 62
#define MMIO_SPTE_GEN_HIGH_MASK GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
MMIO_SPTE_GEN_HIGH_START)
+
static u64 generation_mmio_spte_mask(u64 gen)
{
u64 mask;
WARN_ON(gen & ~MMIO_SPTE_GEN_MASK);
+ BUILD_BUG_ON(MMIO_SPTE_GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT);
mask = (gen << MMIO_SPTE_GEN_LOW_START) & MMIO_SPTE_GEN_LOW_MASK;
mask |= (gen << MMIO_SPTE_GEN_HIGH_START) & MMIO_SPTE_GEN_HIGH_MASK;
--
1.8.3.1
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation
2020-01-21 16:11 [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation Paolo Bonzini
@ 2020-01-21 17:24 ` Ben Gardon
2020-01-21 21:04 ` Sean Christopherson
2020-01-22 14:35 ` Paolo Bonzini
0 siblings, 2 replies; 5+ messages in thread
From: Ben Gardon @ 2020-01-21 17:24 UTC (permalink / raw)
To: Paolo Bonzini; +Cc: linux-kernel, kvm, Sean Christopherson
On Tue, Jan 21, 2020 at 8:11 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> The SPTE_MMIO_MASK overlaps with the bits used to track MMIO
> generation number. A high enough generation number would overwrite the
> SPTE_SPECIAL_MASK region and cause the MMIO SPTE to be misinterpreted;
> likewise, setting bits 52 and 53 would also cause an incorrect generation
> number to be read from the PTE.
>
> Fixes: 6eeb4ef049e7 ("KVM: x86: assign two bits to track SPTE kinds")
> Reported-by: Ben Gardon <bgardon@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 57e4dbddba72..e34ca43d9166 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -418,22 +418,25 @@ static inline bool is_access_track_spte(u64 spte)
> * requires a full MMU zap). The flag is instead explicitly queried when
> * checking for MMIO spte cache hits.
> */
> -#define MMIO_SPTE_GEN_MASK GENMASK_ULL(18, 0)
> +#define MMIO_SPTE_GEN_MASK GENMASK_ULL(17, 0)
I see you're shifting the MMIO high gen mask region to avoid having to
shift it by 2. Looking at the SDM, I believe using bit 62 for the
generation number is safe, but I don't recall why it wasn't used
before.
>
> #define MMIO_SPTE_GEN_LOW_START 3
> #define MMIO_SPTE_GEN_LOW_END 11
> #define MMIO_SPTE_GEN_LOW_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
> MMIO_SPTE_GEN_LOW_START)
>
> -#define MMIO_SPTE_GEN_HIGH_START 52
> -#define MMIO_SPTE_GEN_HIGH_END 61
> +/* Leave room for SPTE_SPECIAL_MASK. */
> +#define MMIO_SPTE_GEN_HIGH_START 54
> +#define MMIO_SPTE_GEN_HIGH_END 62
> #define MMIO_SPTE_GEN_HIGH_MASK GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
> MMIO_SPTE_GEN_HIGH_START)
> +
> static u64 generation_mmio_spte_mask(u64 gen)
> {
> u64 mask;
>
> WARN_ON(gen & ~MMIO_SPTE_GEN_MASK);
> + BUILD_BUG_ON(MMIO_SPTE_GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT);
Would it be worth defining the MMIO_SPTE_GEN masks, SPTE_SPECIAL_MASK,
SPTE_AD masks, and SPTE_MMIO_MASK in terms of
PT64_SECOND_AVAIL_BITS_SHIFT? It seems like that might be a more
robust assertion here.
Alternatively, BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK |
MMIO_SPTE_GEN_LOW_MASK) & SPTE_(MMIO and/or SPECIAL)_MASK)
>
> mask = (gen << MMIO_SPTE_GEN_LOW_START) & MMIO_SPTE_GEN_LOW_MASK;
> mask |= (gen << MMIO_SPTE_GEN_HIGH_START) & MMIO_SPTE_GEN_HIGH_MASK;
> --
> 1.8.3.1
>
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation
2020-01-21 17:24 ` Ben Gardon
@ 2020-01-21 21:04 ` Sean Christopherson
2020-01-23 12:16 ` Paolo Bonzini
2020-01-22 14:35 ` Paolo Bonzini
1 sibling, 1 reply; 5+ messages in thread
From: Sean Christopherson @ 2020-01-21 21:04 UTC (permalink / raw)
To: Ben Gardon; +Cc: Paolo Bonzini, linux-kernel, kvm
On Tue, Jan 21, 2020 at 09:24:07AM -0800, Ben Gardon wrote:
> On Tue, Jan 21, 2020 at 8:11 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > The SPTE_MMIO_MASK overlaps with the bits used to track MMIO
> > generation number. A high enough generation number would overwrite the
> > SPTE_SPECIAL_MASK region and cause the MMIO SPTE to be misinterpreted;
> > likewise, setting bits 52 and 53 would also cause an incorrect generation
> > number to be read from the PTE.
> >
> > Fixes: 6eeb4ef049e7 ("KVM: x86: assign two bits to track SPTE kinds")
> > Reported-by: Ben Gardon <bgardon@google.com>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 9 ++++++---
> > 1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 57e4dbddba72..e34ca43d9166 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -418,22 +418,25 @@ static inline bool is_access_track_spte(u64 spte)
> > * requires a full MMU zap). The flag is instead explicitly queried when
> > * checking for MMIO spte cache hits.
> > */
> > -#define MMIO_SPTE_GEN_MASK GENMASK_ULL(18, 0)
> > +#define MMIO_SPTE_GEN_MASK GENMASK_ULL(17, 0)
>
> I see you're shifting the MMIO high gen mask region to avoid having to
> shift it by 2. Looking at the SDM, I believe using bit 62 for the
> generation number is safe, but I don't recall why it wasn't used
> before.
This patch does use bit 62, see MMIO_SPTE_GEN_HIGH_END below. It's bit
63 that is being avoided because it would collide with NX and EPT suppress
#VE.
> >
> > #define MMIO_SPTE_GEN_LOW_START 3
> > #define MMIO_SPTE_GEN_LOW_END 11
> > #define MMIO_SPTE_GEN_LOW_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
> > MMIO_SPTE_GEN_LOW_START)
> >
> > -#define MMIO_SPTE_GEN_HIGH_START 52
> > -#define MMIO_SPTE_GEN_HIGH_END 61
> > +/* Leave room for SPTE_SPECIAL_MASK. */
> > +#define MMIO_SPTE_GEN_HIGH_START 54
> > +#define MMIO_SPTE_GEN_HIGH_END 62
> > #define MMIO_SPTE_GEN_HIGH_MASK GENMASK_ULL(MMIO_SPTE_GEN_HIGH_END, \
> > MMIO_SPTE_GEN_HIGH_START)
> > +
> > static u64 generation_mmio_spte_mask(u64 gen)
> > {
> > u64 mask;
> >
> > WARN_ON(gen & ~MMIO_SPTE_GEN_MASK);
> > + BUILD_BUG_ON(MMIO_SPTE_GEN_HIGH_START < PT64_SECOND_AVAIL_BITS_SHIFT);
>
> Would it be worth defining the MMIO_SPTE_GEN masks, SPTE_SPECIAL_MASK,
> SPTE_AD masks, and SPTE_MMIO_MASK in terms of
> PT64_SECOND_AVAIL_BITS_SHIFT? It seems like that might be a more
> robust assertion here.
That was Paolo's original proposal, I (successfully) lobbied for using a
BUILG_BUG_ON so that bugs result in a build failure instead of random
runtime issues, e.g. if PT64_SECOND_AVAIL_BITS_SHIFT was changed to a value
that caused the MMIO gen to overlap NX or even shift beyond bit 63.
https://lkml.kernel.org/r/20191212002902.GM5044@linux.intel.com
> Alternatively, BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK |
> MMIO_SPTE_GEN_LOW_MASK) & SPTE_(MMIO and/or SPECIAL)_MASK)
Or add both BUILD_BUG_ONs.
> >
> > mask = (gen << MMIO_SPTE_GEN_LOW_START) & MMIO_SPTE_GEN_LOW_MASK;
> > mask |= (gen << MMIO_SPTE_GEN_HIGH_START) & MMIO_SPTE_GEN_HIGH_MASK;
> > --
> > 1.8.3.1
> >
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation
2020-01-21 17:24 ` Ben Gardon
2020-01-21 21:04 ` Sean Christopherson
@ 2020-01-22 14:35 ` Paolo Bonzini
1 sibling, 0 replies; 5+ messages in thread
From: Paolo Bonzini @ 2020-01-22 14:35 UTC (permalink / raw)
To: Ben Gardon; +Cc: linux-kernel, kvm, Sean Christopherson
On 21/01/20 18:24, Ben Gardon wrote:
> BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK |
> MMIO_SPTE_GEN_LOW_MASK) & SPTE_SPECIAL_MASK)
>
Yes, this is a better assertion. I've replaced it in the patch.
Paolo
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation
2020-01-21 21:04 ` Sean Christopherson
@ 2020-01-23 12:16 ` Paolo Bonzini
0 siblings, 0 replies; 5+ messages in thread
From: Paolo Bonzini @ 2020-01-23 12:16 UTC (permalink / raw)
To: Sean Christopherson, Ben Gardon; +Cc: linux-kernel, kvm
On 21/01/20 22:04, Sean Christopherson wrote:
>> Alternatively, BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK |
>> MMIO_SPTE_GEN_LOW_MASK) & SPTE_(MMIO and/or SPECIAL)_MASK)
> Or add both BUILD_BUG_ONs.
In the end I decided to revert MMIO_SPTE_GEN_HIGH_START to
PT64_SECOND_AVAIL_BITS_SHIFT (it makes sense since we use it for
shadow_acc_track_saved_bits_shift, and whether to use
shadow_acc_track_saved_bits_shift or MMIO_SPTE_GEN_HIGH_START depends on
the SPTE_SPECIAL_MASK bits) and use Ben's suggested BUILD_BUG_ON.
Paolo
^ permalink raw reply [flat|nested] 5+ messages in thread
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2020-01-21 16:11 [PATCH] KVM: x86: fix overlap between SPTE_MMIO_MASK and generation Paolo Bonzini
2020-01-21 17:24 ` Ben Gardon
2020-01-21 21:04 ` Sean Christopherson
2020-01-23 12:16 ` Paolo Bonzini
2020-01-22 14:35 ` Paolo Bonzini
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