powerpc: kvm: make the setup of hpte under the protection of KVMPPC_RMAP_LOCK_BIT
diff mbox

Message ID 1406527744-25316-1-git-send-email-pingfank@linux.vnet.ibm.com
State Rejected
Headers show

Commit Message

Pingfan Liu July 28, 2014, 6:09 a.m. UTC
In current code, the setup of hpte is under the risk of race with
mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

Comments

Benjamin Herrenschmidt July 28, 2014, 6:42 a.m. UTC | #1
On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:
> In current code, the setup of hpte is under the risk of race with
> mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
> Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.

Please describe the race you think you see. I'm quite sure both Paul and
I went over that code and somewhat convinced ourselves that it was ok
but it's possible that we were both wrong :-)

Cheers,
Ben.

> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 8056107..e6dcff4 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -754,19 +754,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>  
>  	if (hptep[0] & HPTE_V_VALID) {
>  		/* HPTE was previously valid, so we need to invalidate it */
> -		unlock_rmap(rmap);
>  		hptep[0] |= HPTE_V_ABSENT;
>  		kvmppc_invalidate_hpte(kvm, hptep, index);
>  		/* don't lose previous R and C bits */
>  		r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
> +
> +		hptep[1] = r;
> +		eieio();
> +		hptep[0] = hpte[0];
> +		asm volatile("ptesync" : : : "memory");
> +		unlock_rmap(rmap);
>  	} else {
> +		hptep[1] = r;
> +		eieio();
> +		hptep[0] = hpte[0];
> +		asm volatile("ptesync" : : : "memory");
>  		kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
>  	}
>  
> -	hptep[1] = r;
> -	eieio();
> -	hptep[0] = hpte[0];
> -	asm volatile("ptesync" : : : "memory");
>  	preempt_enable();
>  	if (page && hpte_is_writable(r))
>  		SetPageDirty(page);
Pingfan Liu July 28, 2014, 7:58 a.m. UTC | #2
Hope I am right.  Take the following seq as an example

if (hptep[0] & HPTE_V_VALID) {
/* HPTE was previously valid, so we need to invalidate it */
unlock_rmap(rmap);
hptep[0] |= HPTE_V_ABSENT;
kvmppc_invalidate_hpte(kvm, hptep, index);
/* don't lose previous R and C bits */
r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
} else {
kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
}
  ---------------------------------------------> if we try_to_unmap on
pfn at here, then @r contains a invalid pfn
hptep[1] = r;
eieio();
hptep[0] = hpte[0];
asm volatile("ptesync" : : : "memory");

Thx.
Fan

On Mon, Jul 28, 2014 at 2:42 PM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:
>> In current code, the setup of hpte is under the risk of race with
>> mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
>> Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.
>
> Please describe the race you think you see. I'm quite sure both Paul and
> I went over that code and somewhat convinced ourselves that it was ok
> but it's possible that we were both wrong :-)
>
> Cheers,
> Ben.
>
>> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>> ---
>>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
>>  1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> index 8056107..e6dcff4 100644
>> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> @@ -754,19 +754,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>>
>>       if (hptep[0] & HPTE_V_VALID) {
>>               /* HPTE was previously valid, so we need to invalidate it */
>> -             unlock_rmap(rmap);
>>               hptep[0] |= HPTE_V_ABSENT;
>>               kvmppc_invalidate_hpte(kvm, hptep, index);
>>               /* don't lose previous R and C bits */
>>               r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
>> +
>> +             hptep[1] = r;
>> +             eieio();
>> +             hptep[0] = hpte[0];
>> +             asm volatile("ptesync" : : : "memory");
>> +             unlock_rmap(rmap);
>>       } else {
>> +             hptep[1] = r;
>> +             eieio();
>> +             hptep[0] = hpte[0];
>> +             asm volatile("ptesync" : : : "memory");
>>               kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
>>       }
>>
>> -     hptep[1] = r;
>> -     eieio();
>> -     hptep[0] = hpte[0];
>> -     asm volatile("ptesync" : : : "memory");
>>       preempt_enable();
>>       if (page && hpte_is_writable(r))
>>               SetPageDirty(page);
>
>
Benjamin Herrenschmidt July 29, 2014, 6:57 a.m. UTC | #3
On Mon, 2014-07-28 at 15:58 +0800, Liu ping fan wrote:
> Hope I am right.  Take the following seq as an example
> 
> if (hptep[0] & HPTE_V_VALID) {
> /* HPTE was previously valid, so we need to invalidate it */
> unlock_rmap(rmap);
> hptep[0] |= HPTE_V_ABSENT;
> kvmppc_invalidate_hpte(kvm, hptep, index);
> /* don't lose previous R and C bits */
> r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
> } else {
> kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
> }
>   ---------------------------------------------> if we try_to_unmap on
> pfn at here, then @r contains a invalid pfn
> hptep[1] = r;
> eieio();
> hptep[0] = hpte[0];
> asm volatile("ptesync" : : : "memory");

If that was the case we would have the same race in kvmppc_do_h_enter().

I think the fact that the HPTE is locked will prevent the race, ie,
HPTE_V_HVLOCK is set until hptep[0] is written to.

If I look at at the unmap case, my understanding is that it uses  
kvm_unmap_rmapp() which will also lock the HPTE (try_lock_hpte)
and so shouldn't have a race vs the above code.

Or do you see a race I don't ?

Cheers,
Ben.

> Thx.
> Fan
> 
> On Mon, Jul 28, 2014 at 2:42 PM, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
> > On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:
> >> In current code, the setup of hpte is under the risk of race with
> >> mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
> >> Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.
> >
> > Please describe the race you think you see. I'm quite sure both Paul and
> > I went over that code and somewhat convinced ourselves that it was ok
> > but it's possible that we were both wrong :-)
> >
> > Cheers,
> > Ben.
> >
> >> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> >> ---
> >>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
> >>  1 file changed, 10 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> >> index 8056107..e6dcff4 100644
> >> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> >> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> >> @@ -754,19 +754,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
> >>
> >>       if (hptep[0] & HPTE_V_VALID) {
> >>               /* HPTE was previously valid, so we need to invalidate it */
> >> -             unlock_rmap(rmap);
> >>               hptep[0] |= HPTE_V_ABSENT;
> >>               kvmppc_invalidate_hpte(kvm, hptep, index);
> >>               /* don't lose previous R and C bits */
> >>               r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
> >> +
> >> +             hptep[1] = r;
> >> +             eieio();
> >> +             hptep[0] = hpte[0];
> >> +             asm volatile("ptesync" : : : "memory");
> >> +             unlock_rmap(rmap);
> >>       } else {
> >> +             hptep[1] = r;
> >> +             eieio();
> >> +             hptep[0] = hpte[0];
> >> +             asm volatile("ptesync" : : : "memory");
> >>               kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
> >>       }
> >>
> >> -     hptep[1] = r;
> >> -     eieio();
> >> -     hptep[0] = hpte[0];
> >> -     asm volatile("ptesync" : : : "memory");
> >>       preempt_enable();
> >>       if (page && hpte_is_writable(r))
> >>               SetPageDirty(page);
> >
> >
Pingfan Liu July 29, 2014, 8:07 a.m. UTC | #4
On Tue, Jul 29, 2014 at 2:57 PM, Benjamin Herrenschmidt
<benh@kernel.crashing.org> wrote:
> On Mon, 2014-07-28 at 15:58 +0800, Liu ping fan wrote:
>> Hope I am right.  Take the following seq as an example
>>
>> if (hptep[0] & HPTE_V_VALID) {
>> /* HPTE was previously valid, so we need to invalidate it */
>> unlock_rmap(rmap);
>> hptep[0] |= HPTE_V_ABSENT;
>> kvmppc_invalidate_hpte(kvm, hptep, index);
>> /* don't lose previous R and C bits */
>> r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
>> } else {
>> kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
>> }
>>   ---------------------------------------------> if we try_to_unmap on
>> pfn at here, then @r contains a invalid pfn
>> hptep[1] = r;
>> eieio();
>> hptep[0] = hpte[0];
>> asm volatile("ptesync" : : : "memory");
>
> If that was the case we would have the same race in kvmppc_do_h_enter().
>
> I think the fact that the HPTE is locked will prevent the race, ie,
> HPTE_V_HVLOCK is set until hptep[0] is written to.
>
> If I look at at the unmap case, my understanding is that it uses
> kvm_unmap_rmapp() which will also lock the HPTE (try_lock_hpte)
> and so shouldn't have a race vs the above code.
>
Yes, you are right :)

Thx,
Fan


> Or do you see a race I don't ?
>
> Cheers,
> Ben.
>
>> Thx.
>> Fan
>>
>> On Mon, Jul 28, 2014 at 2:42 PM, Benjamin Herrenschmidt
>> <benh@kernel.crashing.org> wrote:
>> > On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:
>> >> In current code, the setup of hpte is under the risk of race with
>> >> mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
>> >> Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.
>> >
>> > Please describe the race you think you see. I'm quite sure both Paul and
>> > I went over that code and somewhat convinced ourselves that it was ok
>> > but it's possible that we were both wrong :-)
>> >
>> > Cheers,
>> > Ben.
>> >
>> >> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
>> >> ---
>> >>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
>> >>  1 file changed, 10 insertions(+), 5 deletions(-)
>> >>
>> >> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> >> index 8056107..e6dcff4 100644
>> >> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> >> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> >> @@ -754,19 +754,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
>> >>
>> >>       if (hptep[0] & HPTE_V_VALID) {
>> >>               /* HPTE was previously valid, so we need to invalidate it */
>> >> -             unlock_rmap(rmap);
>> >>               hptep[0] |= HPTE_V_ABSENT;
>> >>               kvmppc_invalidate_hpte(kvm, hptep, index);
>> >>               /* don't lose previous R and C bits */
>> >>               r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
>> >> +
>> >> +             hptep[1] = r;
>> >> +             eieio();
>> >> +             hptep[0] = hpte[0];
>> >> +             asm volatile("ptesync" : : : "memory");
>> >> +             unlock_rmap(rmap);
>> >>       } else {
>> >> +             hptep[1] = r;
>> >> +             eieio();
>> >> +             hptep[0] = hpte[0];
>> >> +             asm volatile("ptesync" : : : "memory");
>> >>               kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
>> >>       }
>> >>
>> >> -     hptep[1] = r;
>> >> -     eieio();
>> >> -     hptep[0] = hpte[0];
>> >> -     asm volatile("ptesync" : : : "memory");
>> >>       preempt_enable();
>> >>       if (page && hpte_is_writable(r))
>> >>               SetPageDirty(page);
>> >
>> >
>
>

Patch
diff mbox

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 8056107..e6dcff4 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -754,19 +754,24 @@  int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
 
 	if (hptep[0] & HPTE_V_VALID) {
 		/* HPTE was previously valid, so we need to invalidate it */
-		unlock_rmap(rmap);
 		hptep[0] |= HPTE_V_ABSENT;
 		kvmppc_invalidate_hpte(kvm, hptep, index);
 		/* don't lose previous R and C bits */
 		r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
+
+		hptep[1] = r;
+		eieio();
+		hptep[0] = hpte[0];
+		asm volatile("ptesync" : : : "memory");
+		unlock_rmap(rmap);
 	} else {
+		hptep[1] = r;
+		eieio();
+		hptep[0] = hpte[0];
+		asm volatile("ptesync" : : : "memory");
 		kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
 	}
 
-	hptep[1] = r;
-	eieio();
-	hptep[0] = hpte[0];
-	asm volatile("ptesync" : : : "memory");
 	preempt_enable();
 	if (page && hpte_is_writable(r))
 		SetPageDirty(page);