[PATCH] KVM: arm64: Retry fault if vma_lookup() results become invalid

[Marc Zyngier]

[Dropping Christoffer's 11 year obsolete address...]

On Mon, 13 Mar 2023 23:54:54 +0000,
David Matlack <dmatlack at google.com> wrote:
> 
> Read mmu_invalidate_seq before dropping the mmap_lock so that KVM can
> detect if the results of vma_lookup() (e.g. vma_shift) become stale
> before it acquires kvm->mmu_lock. This fixes a theoretical bug where a
> VMA could be changed by userspace after vma_lookup() and before KVM
> reads the mmu_invalidate_seq, causing KVM to install page table entries
> based on a (possibly) no-longer-valid vma_shift.
> 
> Re-order the MMU cache top-up to earlier in user_mem_abort() so that it
> is not done after KVM has read mmu_invalidate_seq (i.e. so as to avoid
> inducing spurious fault retries).
> 
> This bug has existed since KVM/ARM's inception. It's unlikely that any
> sane userspace currently modifies VMAs in such a way as to trigger this
> race. And even with directed testing I was unable to reproduce it. But a
> sufficiently motivated host userspace might be able to exploit this
> race.
> 
> Fixes: 94f8e6418d39 ("KVM: ARM: Handle guest faults in KVM")

Ah, good luck with that one! :D user_mem_abort() used to be so nice
and simple at the time! And yet...

> Cc: stable at vger.kernel.org
> Reported-by: Sean Christopherson <seanjc at google.com>
> Signed-off-by: David Matlack <dmatlack at google.com>

Reviewed-by: Marc Zyngier <maz at kernel.org>

Oliver, how do you want to deal with this one? queue it right now? Or
wait until the dust settles on my two other patches?

I don't mind either way, I can either take it as part of the same
series, or rebase my stuff on it.

Thanks,

	M.

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