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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);
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