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

Nick Desaulniers
2020-03-23T18:49:16Z

On Mon, Mar 23, 2020 at 11:16 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> 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)

Sean said:
> 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.

I'm not sure about this, I think ASM_CALL_CONSTRAINT may be a red
herring, based on the commit message that added it (commit
f5caf621ee357 ("x86/asm: Fix inline asm call constraints for Clang")).

Further, it seems the "m" in "rm" in THUNK_TARGET for
CONFIG_RETPOLINE=n is problematic.

THUNK_TARGET defines [thunk_target] as "rm" when CONFIG_RETPOLINE is
not set, which isn't constrained enough for this specific case; if
`entry` winds up at the bottom of the stack where rsp points to, then
`%rsp` is good enough to satisfy the constraints for using `entry` as
an input. For inline assembly that modifies the the stack pointer
before using this input, the underspecification of constraints is
dangerous, and results in an indirect call to a previously pushed
flags register.

So maybe we can find why
commit 76b043848fd2 ("x86/retpoline: Add initial retpoline support")
added THUNK_TARGET with and without "m" constraint, and either:
- remove "m" from THUNK_TARGET. (Maybe this doesn't compile somewhere)
or
- use my above recommendation locally avoiding THUNK_TARGET. We can
use "r" rather than "a" (what Clang would have picked) or "b (what GCC
would have picked) to give the compilers maximal flexibility.
--
Thanks,
~Nick Desaulniers