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

[Eduard Zingerman <eddyz87@gmail.com>]

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;