linux-kernel - Re: Re: WARNING in x86_emulate

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Date:   Tue, 12 Dec 2017 16:50:11 +0800
From:   Lan Tianyu <tianyu.lan@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Tianyu Lan <lantianyu1986@...il.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot 
        <bot+75375385991b4f8c599704a10849863c586ea284@...kaller.appspotmail.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        syzkaller-bugs@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: Re: WARNING in x86_emulate_insn

On 2017年12月12日 06:45, Paolo Bonzini wrote:
> On 08/12/2017 09:28, Tianyu Lan wrote:
>> I find this is pop instruction emulation issue. According "SDM VOL2,
>> chapter INSTRUCTION
>> SET REFERENCE. POP—Pop a Value from the Stack"
>>
>> Protected Mode Exceptions
>> #GP(0) If attempt is made to load SS register with NULL segment selector.
> 
> This is not what the testcase is testing; this is already covered by 
> __load_segment_descriptor:
> 
>         if (null_selector) {
>                 if (seg == VCPU_SREG_CS || seg == VCPU_SREG_TR)
>                         goto exception;
> 
>                 if (seg == VCPU_SREG_SS) {
>                         if (ctxt->mode != X86EMUL_MODE_PROT64 || rpl != cpl)
>                                 goto exception;
> 		...
> 	}

Yes, __load_segment_descriptor() does such check. I find em_pop doesn't
load SS segment. SS isn't loaded before calling em_pop in the test case.
Should this be fixed?

> 
> Is there a path that can return X86EMUL_PROPAGATE_FAULT without setting
> ctxt->exception.vector and/or without going through emulate_exception?
> 
> I don't think it's possible to write a test in kvm-unit-tests, because the
> state has "impossible" segment descriptor cache contents.

Sent out a fix patch for the issue. Please have a look. Thanks.
https://marc.info/?l=kvm&m=151306208214733&w=2

> 
> Paolo
> 
>> This test case hits it but current code doesn't check such case.
>> The following patch can fix the issue.
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index abe74f7..e2ac5cc 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -1844,6 +1844,9 @@ static int emulate_pop(struct x86_emulate_ctxt *ctxt,
>>         int rc;
>>         struct segmented_address addr;
>>
>> +       if ( !get_segment_selector(ctxt, VCPU_SREG_SS))
>> +               return emulate_gp(ctxt, 0);
>> +
>>         addr.ea = reg_read(ctxt, VCPU_REGS_RSP) & stack_mask(ctxt);
>>         addr.seg = VCPU_SREG_SS;
>>         rc = segmented_read(ctxt, addr, dest, len);
> 


-- 
Best regards
Tianyu Lan