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

[Alexander Potapenko]

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.