On Wed, Apr 3, 2024 at 2:40 PM John Fastabend <john.fastabend@xxxxxxxxx> wrote:
>
> Tao Lyu wrote:
> > The verifier previously aimed to reject partial overwrite on an 8-byte stack slot
> > that contains a spilled pointer.
> > However, it rejects all partial stack overwrites
> > as long as the targeted stack slot is a spilled register,
> > because it does not check if the stack slot is a spilled pointer.
> >
> > Finally, incomplete checks will result in the rejection of valid programs,
> > which spill narrower scalar values onto scalar slots, as shown below.
> >
> > 0: R1=ctx() R10=fp0
> > ; asm volatile ( @ repro.bpf.c:679
> > 0: (7a) *(u64 *)(r10 -8) = 1 ; R10=fp0 fp-8_w=1
> > 1: (62) *(u32 *)(r10 -8) = 1
> > attempt to corrupt spilled pointer on stack
> > processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0.
> >
> > The issue is fixed by adding a check on the spilled register type of targeted slots.
> >
> > Fixes: ab125ed3ec1c ("bpf: fix check for attempt to corrupt spilled pointer")
> > Signed-off-by: Tao Lyu <tao.lyu@xxxxxxx>
> > ---
>
> A test case would be nice.
+1, please see progs/verifier_spill_fill.c, seems like a logical place
>
> > kernel/bpf/verifier.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 17c06f1505e4..3064ba7c140f 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -4493,6 +4493,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
> > */
> > if (!env->allow_ptr_leaks &&
> > is_spilled_reg(&state->stack[spi]) &&
> > + state->stack[spi].spilled_ptr.type != SCALAR_VALUE &&
> > size != BPF_REG_SIZE) {
>
> Might be worth a helper we have is_spilled_reg and is_spilled_scalar_reg
> maybe a is_spilled_not_scalar_reg() is nice. Perhaps Andrii has a style
> opinion. Or,
>
> is_spilled_reg(...) && !is_spilled_scalar_reg(...)
yeah, I'd probably go with this, definitely not "is_spilled_not_scalar_reg" :)
>
> Not sure I like that any better than whats there in patch though.
>
> > verbose(env, "attempt to corrupt spilled pointer on stack\n");
> > return -EACCES;
> > --
> > 2.25.1
> >
> >
>
>
