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

[Nick Desaulniers]

On Mon, Mar 23, 2020 at 10:55 AM Alexander Potapenko <glider@xxxxxxxxxx> 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 think this reproducer demonstrates the issue:
https://godbolt.org/z/jAafjz
I *think* what's happening is that we're not specifying correctly that
the stack is being modified by inline asm, so using variable
references against the stack pointer is not correct.

commit f5caf621ee357 ("x86/asm: Fix inline asm call constraints for Clang")
has more context about ASM_CALL_CONSTRAINT.

It seems that specifying "rsp" in the clobber list is a -Wdeprecated
warning in GCC, and an error in Clang (unless you remove
current_stack_pointer as an output, but will get Clang to produce the
correct code).

-- 
Thanks,
~Nick Desaulniers