linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference in handle_external_interrupt

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Date:   Mon, 23 Mar 2020 09:39:25 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+3f29ca2efb056a761e38@...kaller.appspotmail.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, wanpengli@...cent.com,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in
 handle_external_interrupt_irqoff

On Mon, Mar 23, 2020 at 05:31:15PM +0100, Alexander Potapenko wrote:
> On Mon, Mar 23, 2020 at 9:18 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >
> > On 22/03/20 07:59, Dmitry Vyukov wrote:
> > >
> > > The commit range is presumably
> > > fb279f4e238617417b132a550f24c1e86d922558..63849c8f410717eb2e6662f3953ff674727303e7
> > > But I don't see anything that says "it's me". The only commit that
> > > does non-trivial changes to x86/vmx seems to be "KVM: VMX: check
> > > descriptor table exits on instruction emulation":
> >
> > That seems unlikely, it's a completely different file and it would only
> > affect the outside (non-nested) environment rather than your own kernel.
> >
> > The only instance of "0x86" in the registers is in the flags:
> >
> > > RSP: 0018:ffffc90001ac7998 EFLAGS: 00010086
> > > RAX: ffffc90001ac79c8 RBX: fffffe0000000000 RCX: 0000000000040000
> > > RDX: ffffc9000e20f000 RSI: 000000000000b452 RDI: 000000000000b453
> > > RBP: 0000000000000ec0 R08: ffffffff83987523 R09: ffffffff811c7eca
> > > R10: ffff8880a4e94200 R11: 0000000000000002 R12: dffffc0000000000
> > > R13: fffffe0000000ec8 R14: ffffffff880016f0 R15: fffffe0000000ecb
> > > FS:  00007fb50e370700(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 000000000000005c CR3: 0000000092fc7000 CR4: 00000000001426f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >
> > That would suggest a miscompilation of the inline assembly, which does
> > push the flags:
> >
> > #ifdef CONFIG_X86_64
> >                 "mov %%" _ASM_SP ", %[sp]\n\t"
> >                 "and $0xfffffffffffffff0, %%" _ASM_SP "\n\t"
> >                 "push $%c[ss]\n\t"
> >                 "push %[sp]\n\t"
> > #endif
> >                 "pushf\n\t"
> >                 __ASM_SIZE(push) " $%c[cs]\n\t"
> >                 CALL_NOSPEC
> >
> >
> > It would not explain why it suddenly started to break, unless the clang
> > version also changed, but it would be easy to ascertain and fix (in
> > either KVM or clang).  Dmitry, can you send me the vmx.o and
> > kvm-intel.ko files?
> 
> On a quick glance, Clang does not miscompile this part.

Clang definitely miscompiles the asm, the indirect call operates on the
EFLAGS value, not on @entry as expected.  It looks like clang doesn't honor
ASM_CALL_CONSTRAINT, which effectively tells the compiler that %rsp is
getting clobbered, e.g. the "mov %r14,0x8(%rsp)" is loading @entry for
"callq *0x8(%rsp)", which breaks because of asm's pushes.

clang:

	kvm_before_interrupt(vcpu);

	asm volatile(
ffffffff811b798e:	4c 89 74 24 08       	mov    %r14,0x8(%rsp)
ffffffff811b7993:	48 89 e0             	mov    %rsp,%rax
ffffffff811b7996:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
ffffffff811b799a:	6a 18                	pushq  $0x18
ffffffff811b799c:	50                   	push   %rax
ffffffff811b799d:	9c                   	pushfq 
ffffffff811b799e:	6a 10                	pushq  $0x10
ffffffff811b79a0:	ff 54 24 08          	callq  *0x8(%rsp) <--------- calls the EFLAGS value
kvm_after_interrupt():


gcc:
	kvm_before_interrupt(vcpu);

	asm volatile(
ffffffff8118e17c:	48 89 e0             	mov    %rsp,%rax
ffffffff8118e17f:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
ffffffff8118e183:	6a 18                	pushq  $0x18
ffffffff8118e185:	50                   	push   %rax
ffffffff8118e186:	9c                   	pushfq 
ffffffff8118e187:	6a 10                	pushq  $0x10
ffffffff8118e189:	ff d3                	callq  *%rbx <-------- calls @entry
kvm_after_interrupt():