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

Nick Desaulniers
2020-03-23T16:57:49Z

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
> 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