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

Ryan Roberts
2022-12-05T14:07:37Z

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