linux-kernel - Re: [PATCH v2 7/8] KVM: x86: Bug the VM if the emulator generates a bogus exception vector

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Date:   Wed, 01 Jun 2022 10:31:49 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Robert Dinse <nanook@...imo.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 7/8] KVM: x86: Bug the VM if the emulator generates a
 bogus exception vector

Sean Christopherson <seanjc@...gle.com> writes:

> Bug the VM if KVM's emulator attempts to inject a bogus exception vector.
> The guest is likely doomed even if KVM continues on, and propagating a
> bad vector to the rest of KVM runs the risk of breaking other assumptions
> in KVM and thus triggering a more egregious bug.
>
> All existing users of emulate_exception() have hardcoded vector numbers
> (__load_segment_descriptor() uses a few different vectors, but they're
> all hardcoded), and future users are likely to follow suit, i.e. the
> change to emulate_exception() is a glorified nop.
>
> As for the ctxt->exception.vector check in x86_emulate_insn(), the few
> known times the WARN has been triggered in the past is when the field was
> not set when synthesizing a fault, i.e. for all intents and purposes the
> check protects against consumption of uninitialized data.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/emulate.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 70a8e0cd9fdc..2aa17462a9ac 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -624,7 +624,9 @@ static unsigned long seg_base(struct x86_emulate_ctxt *ctxt, int seg)
>  static int emulate_exception(struct x86_emulate_ctxt *ctxt, int vec,
>  			     u32 error, bool valid)
>  {
> -	WARN_ON(vec > 0x1f);
> +	if (KVM_EMULATOR_BUG_ON(vec > 0x1f, ctxt))
> +		return X86EMUL_UNHANDLEABLE;
> +
>  	ctxt->exception.vector = vec;
>  	ctxt->exception.error_code = error;
>  	ctxt->exception.error_code_valid = valid;
> @@ -5728,7 +5730,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
>  
>  done:
>  	if (rc == X86EMUL_PROPAGATE_FAULT) {
> -		WARN_ON(ctxt->exception.vector > 0x1f);
> +		if (KVM_EMULATOR_BUG_ON(ctxt->exception.vector > 0x1f, ctxt))
> +			return EMULATION_FAILED;
>  		ctxt->have_exception = true;
>  	}
>  	if (rc == X86EMUL_INTERCEPTED)

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly