Re: BUG: unable to handle kernel NULL pointer dereference in handle_external_interrupt_irqoff

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Mar 23, 2020 at 9:57 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Mon, Mar 23, 2020 at 9:39 AM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > 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@redhat.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

I noticed that in the syzcaller config I have, that CONFIG_RETPOLINE
is not set.  I'm more reliably able to reproduce this with
clang+defconfig+CONFIG_KVM=y+CONFIG_KVM_INTEL=y+CONFIG_RETPOLINE=n,
ie. by manually disabling retpoline.

> > 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():
>
> Thanks for this analysis, it looks like this is dependent on some
> particular configuration; here's clang+defconfig+CONFIG_KVM_INTEL=y:
>
>    0x000000000000528f <+127>:   pushq  $0x18
>    0x0000000000005291 <+129>:   push   %rcx
>    0x0000000000005292 <+130>:   pushfq
>    0x0000000000005293 <+131>:   pushq  $0x10
>    0x0000000000005295 <+133>:   callq  *%rax
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers