linux-kernel - Re: WARNING in x86_emulate

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ec6ee88f-1d56-9bb9-53a9-41e4c9e800d8@redhat.com>
Date:   Mon, 11 Dec 2017 23:45:57 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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: WARNING in x86_emulate_insn

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;
		...
	}

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.

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);