linux-kernel - Re: [PATCH 2/3] KVM: x86: Set error code to segment selector on LLDT/LTR non-canonical #GP

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Date:   Tue, 12 Jul 2022 16:37:06 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+760a73552f47a8cd0fd9@...kaller.appspotmail.com,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Hou Wenlong <houwenlong.hwl@...group.com>
Subject: Re: [PATCH 2/3] KVM: x86: Set error code to segment selector on
 LLDT/LTR non-canonical #GP

On Mon, 2022-07-11 at 23:27 +0000, Sean Christopherson wrote:
> When injecting a #GP on LLDT/LTR due to a non-canonical LDT/TSS base, set
> the error code to the selector.  Intel SDM's says nothing about the #GP,
> but AMD's APM explicitly states that both LLDT and LTR set the error code
> to the selector, not zero.
> 
> Note, a non-canonical memory operand on LLDT/LTR does generate a #GP(0),
> but the KVM code in question is specific to the base from the descriptor.
> 
> Fixes: e37a75a13cda ("KVM: x86: Emulator ignores LDTR/TR extended base on LLDT/LTR")
> Cc: stable@...r.kernel.org
> Signed-off-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 09e4b67b881f..bd9e9c5627d0 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1736,8 +1736,8 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
>                 if (ret != X86EMUL_CONTINUE)
>                         return ret;
>                 if (emul_is_noncanonical_address(get_desc_base(&seg_desc) |
> -                               ((u64)base3 << 32), ctxt))
> -                       return emulate_gp(ctxt, 0);
> +                                                ((u64)base3 << 32), ctxt))
> +                       return emulate_gp(ctxt, err_code);
>         }
>  
>         if (seg == VCPU_SREG_TR) {

I guess this is the quote from AMD's manual (might we worth to add to the source?)


"The 64-bit system-segment base address must be in canonical form. Otherwise, a general-protection
exception occurs with a selector error-code, #GP(selector), when the system segment is loaded.
System-segment limit values are checked by the processor in both 64-bit and compatibility modes,
under the control of the granularity (G) bit."

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

Best regards,
	Maxim Levitsky