Re: [PATCH] KVM: arm64: Avoid corrupting vCPU context register in guest exit

From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org, kernel-team@android.com,
	Andrew Scull <ascull@google.com>,
	stable@vger.kernel.org, Quentin Perret <qperret@google.com>
Subject: Re: [PATCH] KVM: arm64: Avoid corrupting vCPU context register in guest exit
Date: Fri, 26 Feb 2021 18:35:42 +0000	[thread overview]
Message-ID: <f6b11e03ac9f0a4830e0a8f56a91450f@kernel.org> (raw)
In-Reply-To: <20210226181211.14542-1-will@kernel.org>

On 2021-02-26 18:12, Will Deacon wrote:
> Commit 7db21530479f ("KVM: arm64: Restore hyp when panicking in guest
> context") tracks the currently running vCPU, clearing the pointer to
> NULL on exit from a guest.
> 
> Unfortunately, the use of 'set_loaded_vcpu' clobbers x1 to point at the
> kvm_hyp_ctxt instead of the vCPU context, causing the subsequent RAS
> code to go off into the weeds when it saves the DISR assuming that the
> CPU context is embedded in a struct vCPU.
> 
> Leave x1 alone and use x3 as a temporary register instead when clearing
> the vCPU on the guest exit path.
> 
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Andrew Scull <ascull@google.com>
> Cc: <stable@vger.kernel.org>
> Fixes: 7db21530479f ("KVM: arm64: Restore hyp when panicking in guest 
> context")
> Suggested-by: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> 
> This was pretty awful to debug!
> 
>  arch/arm64/kvm/hyp/entry.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index b0afad7a99c6..0c66a1d408fd 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -146,7 +146,7 @@ SYM_INNER_LABEL(__guest_exit, SYM_L_GLOBAL)
>  	// Now restore the hyp regs
>  	restore_callee_saved_regs x2
> 
> -	set_loaded_vcpu xzr, x1, x2
> +	set_loaded_vcpu xzr, x2, x3
> 
>  alternative_if ARM64_HAS_RAS_EXTN
>  	// If we have the RAS extensions we can consume a pending error

Grmbl... How comes we have never seen that for the past 5 months,
including on CPUs that implement RAS?

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...