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

From: Marc Zyngier <maz@kernel.org>
To: David Matlack <dmatlack@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
	kvm@vger.kernel.org, James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>, Will Deacon <will@kernel.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
	stable@vger.kernel.org, Sean Christopherson <seanjc@google.com>
Subject: Re: [PATCH] KVM: arm64: Retry fault if vma_lookup() results become invalid
Date: Tue, 14 Mar 2023 16:31:38 +0000	[thread overview]
Message-ID: <86fsa7xpjp.wl-maz@kernel.org> (raw)
In-Reply-To: <20230313235454.2964067-1-dmatlack@google.com>

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

On Mon, 13 Mar 2023 23:54:54 +0000,
David Matlack <dmatlack@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@vger.kernel.org
> Reported-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: David Matlack <dmatlack@google.com>

Reviewed-by: Marc Zyngier <maz@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.