linux-kernel - Re: [PATCH v2 06/11] KVM: x86: emulator/smm: number of GPRs in the SMRAM image depends on the image format

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Message-ID: <YtiYdTWQ7Vy+IHLO@google.com>
Date:   Thu, 21 Jul 2022 00:06:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 06/11] KVM: x86: emulator/smm: number of GPRs in the
 SMRAM image depends on the image format

On Tue, Jun 21, 2022, Maxim Levitsky wrote:
> On 64 bit host, if the guest doesn't have X86_FEATURE_LM, we would

s/we would/KVM will

> access 16 gprs to 32-bit smram image, causing out-ouf-bound ram
> access.
> 
> On 32 bit host, the rsm_load_state_64/enter_smm_save_state_64
> is compiled out, thus access overflow can't happen.
> 
> Fixes: b443183a25ab61 ("KVM: x86: Reduce the number of emulator GPRs to '8' for 32-bit KVM")

Argh, I forgot that this one of the like five places KVM actually respects the
long mode flag.  Even worse, I fixed basically the same thing a while back,
commit b68f3cc7d978 ("KVM: x86: Always use 32-bit SMRAM save state for 32-bit kernels").

We should really harden put_smstate() and GET_SMSTATE()...

> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---

Nits aside,

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

>  arch/x86/kvm/emulate.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 002687d17f9364..ce186aebca8e83 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2469,7 +2469,7 @@ static int rsm_load_state_32(struct x86_emulate_ctxt *ctxt,
>  	ctxt->eflags =             GET_SMSTATE(u32, smstate, 0x7ff4) | X86_EFLAGS_FIXED;
>  	ctxt->_eip =               GET_SMSTATE(u32, smstate, 0x7ff0);
>  
> -	for (i = 0; i < NR_EMULATOR_GPRS; i++)
> +	for (i = 0; i < 8; i++)
>  		*reg_write(ctxt, i) = GET_SMSTATE(u32, smstate, 0x7fd0 + i * 4);
>  
>  	val = GET_SMSTATE(u32, smstate, 0x7fcc);
> @@ -2526,7 +2526,7 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
>  	u16 selector;
>  	int i, r;
>  
> -	for (i = 0; i < NR_EMULATOR_GPRS; i++)
> +	for (i = 0; i < 16; i++)
>  		*reg_write(ctxt, i) = GET_SMSTATE(u64, smstate, 0x7ff8 - i * 8);
>  
>  	ctxt->_eip   = GET_SMSTATE(u64, smstate, 0x7f78);
> -- 
> 2.26.3
>