[PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.

[PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.

[Ryan Roberts]

get_user_mapping_size() uses kvm's pgtable library to walk a user space
page table created by the kernel, and in doing so, fakes up the metadata
that the library needs, including ia_bits, which defines the size of the
input address.

For the case where the kernel is compiled for 52 VA bits but runs on HW
that does not support LVA, it will fall back to 48 VA bits at runtime.
Therefore we must use vabits_actual rather than VA_BITS to get the true
address size.

This is benign in the current code base because the pgtable library only
uses it for error checking.

Fixes: 6011cf68c885 ("KVM: arm64: Walk userspace page tables to compute
the THP mapping size")

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 arch/arm64/kvm/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 4efb983cff43..1ef0704420d9 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -641,7 +641,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
 {
 	struct kvm_pgtable pgt = {
 		.pgd		= (kvm_pte_t *)kvm->mm->pgd,
-		.ia_bits	= VA_BITS,
+		.ia_bits	= vabits_actual,
 		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
 				   CONFIG_PGTABLE_LEVELS),
 		.mm_ops		= &kvm_user_mm_ops,
--
2.25.1


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Re: [PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.

[Marc Zyngier]

Hi Ryan,

Thanks for that.

On Mon, 05 Dec 2022 11:40:31 +0000,
Ryan Roberts <ryan.roberts@arm.com> wrote:
> 
> get_user_mapping_size() uses kvm's pgtable library to walk a user space
> page table created by the kernel, and in doing so, fakes up the metadata
> that the library needs, including ia_bits, which defines the size of the
> input address.

It isn't supposed to "fake" anything. It simply provides the
information that the walker needs to correctly parse the page tables.

> 
> For the case where the kernel is compiled for 52 VA bits but runs on HW
> that does not support LVA, it will fall back to 48 VA bits at runtime.
> Therefore we must use vabits_actual rather than VA_BITS to get the true
> address size.
> 
> This is benign in the current code base because the pgtable library only
> uses it for error checking.
> 
> Fixes: 6011cf68c885 ("KVM: arm64: Walk userspace page tables to compute
> the THP mapping size")

nit: this should appear on a single line, without a line-break in the
middle [1]...

>

... without a blank line between Fixes: and the rest of the tags.

And while I'm on the "trivial remarks" train, drop the full stop at
the end of the subject line.

> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  arch/arm64/kvm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 4efb983cff43..1ef0704420d9 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -641,7 +641,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
>  {
>  	struct kvm_pgtable pgt = {
>  		.pgd		= (kvm_pte_t *)kvm->mm->pgd,
> -		.ia_bits	= VA_BITS,
> +		.ia_bits	= vabits_actual,
>  		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
>  				   CONFIG_PGTABLE_LEVELS),
>  		.mm_ops		= &kvm_user_mm_ops,
> --
> 2.25.1
> 
> 

Other than the above nits, this is well spotted. I need to regenerate
the kvmarm/next branch after the sysreg attack from James, so I'll try
and fold that in.

Thanks,

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst#n139

-- 
Without deviation from the norm, progress is not possible.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Re: [PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.

[Ryan Roberts]

On 05/12/2022 13:49, Marc Zyngier wrote:
> Hi Ryan,
> 
> Thanks for that.
> 
> On Mon, 05 Dec 2022 11:40:31 +0000,
> Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> get_user_mapping_size() uses kvm's pgtable library to walk a user space
>> page table created by the kernel, and in doing so, fakes up the metadata
>> that the library needs, including ia_bits, which defines the size of the
>> input address.
> 
> It isn't supposed to "fake" anything. It simply provides the
> information that the walker needs to correctly parse the page tables.

Apologies - poor choice of words.

> 
>>
>> For the case where the kernel is compiled for 52 VA bits but runs on HW
>> that does not support LVA, it will fall back to 48 VA bits at runtime.
>> Therefore we must use vabits_actual rather than VA_BITS to get the true
>> address size.
>>
>> This is benign in the current code base because the pgtable library only
>> uses it for error checking.
>>
>> Fixes: 6011cf68c885 ("KVM: arm64: Walk userspace page tables to compute
>> the THP mapping size")
> 
> nit: this should appear on a single line, without a line-break in the
> middle [1]...>
>>
> 
> ... without a blank line between Fixes: and the rest of the tags.

Ahh, thanks for the pointer. I'll admit that checkpatch did raise this but I
assumed it was a false positive, because assumed the 75 chars per line rule
would override.

> 
> And while I'm on the "trivial remarks" train, drop the full stop at
> the end of the subject line.

Yep, will do.

> 
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>  arch/arm64/kvm/mmu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index 4efb983cff43..1ef0704420d9 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -641,7 +641,7 @@ static int get_user_mapping_size(struct kvm *kvm, u64 addr)
>>  {
>>  	struct kvm_pgtable pgt = {
>>  		.pgd		= (kvm_pte_t *)kvm->mm->pgd,
>> -		.ia_bits	= VA_BITS,
>> +		.ia_bits	= vabits_actual,
>>  		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
>>  				   CONFIG_PGTABLE_LEVELS),
>>  		.mm_ops		= &kvm_user_mm_ops,
>> --
>> 2.25.1
>>
>>
> 
> Other than the above nits, this is well spotted. I need to regenerate
> the kvmarm/next branch after the sysreg attack from James, so I'll try
> and fold that in.

Sounds like you are happy to tend to the nits and don't need me to repost?

> 
> Thanks,
> 
> 	M.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst#n139
> 


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

Re: [PATCH v1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.

[Marc Zyngier]

On Mon, 5 Dec 2022 11:40:31 +0000, Ryan Roberts wrote:
> get_user_mapping_size() uses kvm's pgtable library to walk a user space
> page table created by the kernel, and in doing so, fakes up the metadata
> that the library needs, including ia_bits, which defines the size of the
> input address.
> 
> For the case where the kernel is compiled for 52 VA bits but runs on HW
> that does not support LVA, it will fall back to 48 VA bits at runtime.
> Therefore we must use vabits_actual rather than VA_BITS to get the true
> address size.
> 
> [...]

Applied to next, thanks!

[1/1] KVM: arm64: Fix benign bug with incorrect use of VA_BITS.
      commit: 219072c09abde0f1d0a6ce091be375e8eb7d08f0

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.



_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel