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

Nick Desaulniers
2020-03-23T18:16:29Z

On Mon, Mar 23, 2020 at 11:06 AM Alexander Potapenko <glider@google.com> wrote:
>
> On Mon, Mar 23, 2020 at 6:55 PM Alexander Potapenko <glider@google.com> wrote:
> >
> > I've reduced the faulty test case to the following code:
> >
> > =================================
> > a;
> > long b;
> > register unsigned long current_stack_pointer asm("rsp");
> > handle_external_interrupt_irqoff() {
> > asm("and $0xfffffffffffffff0, %%rsp\n\tpush $%c[ss]\n\tpush "
> > "%[sp]\n\tpushf\n\tpushq $%c[cs]\n\tcall *%[thunk_target]\n"
> > : [ sp ] "=&r"(b), "+r" (current_stack_pointer)
> > : [ thunk_target ] "rm"(a), [ ss ] "i"(3 * 8), [ cs ] "i"(2 * 8) );
> > }
> > =================================
> > (in fact creduce even throws away current_stack_pointer, but we
> > probably want to keep it to prove the point).
> >
> > Clang generates the following code for it:
> >
> > $ clang vmx.i -O2 -c -w -o vmx.o
> > $ objdump -d vmx.o
> > ...
> > 0000000000000000 <handle_external_interrupt_irqoff>:
> > 0: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 6
> > <handle_external_interrupt_irqoff+0x6>
> > 6: 89 44 24 fc mov %eax,-0x4(%rsp)
> > a: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
> > e: 6a 18 pushq $0x18
> > 10: 50 push %rax
> > 11: 9c pushfq
> > 12: 6a 10 pushq $0x10
> > 14: ff 54 24 fc callq *-0x4(%rsp)
> > 18: 48 89 05 00 00 00 00 mov %rax,0x0(%rip) # 1f
> > <handle_external_interrupt_irqoff+0x1f>
> > 1f: c3 retq
> >
> > The question is whether using current_stack_pointer as an output is
> > actually a valid way to tell the compiler it should not clobber RSP.
> > Intuitively it is, but explicitly adding RSP to the clobber list
> > sounds a bit more bulletproof.
>
> Ok, I am wrong: according to
> https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html it's incorrect to
> list RSP in the clobber list.

You could force `entry` into a register:
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 4d22b1b5e822..083a7e980bb5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6277,7 +6277,7 @@ static void
handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
#endif
ASM_CALL_CONSTRAINT
:
- THUNK_TARGET(entry),
+ [thunk_target] "a"(entry),
[ss]"i"(__KERNEL_DS),
[cs]"i"(__KERNEL_CS)
);

(https://stackoverflow.com/a/48877683/1027966 had some interesting
feedback to this problem)
--
Thanks,
~Nick Desaulniers