linux-kernel - Re: [RESEND PATCH] kvm/x86: propagate fetch fault into guest

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Date:   Fri, 11 Jan 2019 14:30:03 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Peng Hao <peng.hao2@....com.cn>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH]  kvm/x86: propagate fetch fault into guest

2018-12-24 20:00+0800, Peng Hao:
> When handling ept misconfig exit, it will call emulate instruction
> with insn_len = 0. The decode instruction function may return a fetch
> fault and should propagate to guest.
> 
> The problem will result to emulation fail.
> KVM internal error. Suberror: 1
> emulation failure
> EAX=f81a0024 EBX=f6a07000 ECX=f6a0737c EDX=f8be0118
> ESI=f6a0737c EDI=00000021 EBP=f6929f98 ESP=f6929f98
> EIP=f8bdd141 EFL=00010086 [--S--P-] CPL=0 II=0 A20=1 SMM=0 HLT=0
> ES =007b 00000000 ffffffff 00c0f300 DPL=3 DS   [-WA]
> CS =0060 00000000 ffffffff 00c09b00 DPL=0 CS32 [-RA]
> SS =0068 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
> DS =007b 00000000 ffffffff 00c0f300 DPL=3 DS   [-WA]
> FS =00d8 2c044000 ffffffff 00809300 DPL=0 DS16 [-WA]
> GS =0033 081a44c8 01000fff 00d0f300 DPL=3 DS   [-WA]
> LDT=0000 00000000 ffffffff 00000000
> TR =0080 f6ea0c80 0000206b 00008b00 DPL=0 TSS32-busy
> GDT=     f6e99000 000000ff
> IDT=     fffbb000 000007ff
> CR0=80050033 CR2=b757d000 CR3=35d31000 CR4=001406d0

Do you have a test case for this?

> Signed-off-by: Peng Hao <peng.hao2@....com.cn>
> ---
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> @@ -5114,8 +5114,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
>  		memcpy(ctxt->fetch.data, insn, insn_len);
>  	else {
>  		rc = __do_insn_fetch_bytes(ctxt, 1);
> -		if (rc != X86EMUL_CONTINUE)
> +		if (rc != X86EMUL_CONTINUE) {
> +			if (rc == X86EMUL_PROPAGATE_FAULT)
> +				ctxt->have_exception = true;
>  			return rc;

(Ugh, the caller expects EMULATION_FAILED instead of rc.)

> +		}
>  	}
>  
>  	switch (mode) {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -6333,8 +6333,10 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>  			if (reexecute_instruction(vcpu, cr2, write_fault_to_spt,
>  						emulation_type))
>  				return EMULATE_DONE;
> -			if (ctxt->have_exception && inject_emulated_exception(vcpu))

I don't understand what that return value check was supposed to do, but
yours version seems good.

I have queued it for rc3 to get some extra testing,

thanks.