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

Alexander Potapenko
2020-03-23T17:55:21Z

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.