Re: [PATCH bpf-next 4/7] bpf: fix check for attempt to corrupt spilled pointer

Eduard Zingerman
2023-11-09T15:20:44Z

On Mon, 2023-10-30 at 22:03 -0700, Andrii Nakryiko wrote:
> > When register is spilled onto a stack as a 1/2/4-byte register, we set
> > slot_type[BPF_REG_SIZE - 1] (plus potentially few more below it,
> > depending on actual spill size). So to check if some stack slot has
> > spilled register we need to consult slot_type[7], not slot_type[0].
> >
> > To avoid the need to remember and double-check this in the future, just
> > use is_spilled_reg() helper.
> >
> > Fixes: 638f5b90d460 ("bpf: reduce verifier memory consumption")
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

> > ---
> > kernel/bpf/verifier.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 098ba0e1a6ff..82992c32c1bd 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -4622,7 +4622,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> > * so it's aligned access and [off, off + size) are within stack limits
> > */
> > if (!env->allow_ptr_leaks &&
> > - state->stack[spi].slot_type[0] == STACK_SPILL &&
> > + is_spilled_reg(&state->stack[spi]) &&
> > size != BPF_REG_SIZE) {
> > verbose(env, "attempt to corrupt spilled pointer on stack\n");
> > return -EACCES;