Re: [PATCH v4 bpf-next 08/14] bpf: introduce the bpf_get_local_storage() helper function

[Roman Gushchin <guro@fb.com>]

On Wed, Aug 01, 2018 at 12:50:16AM +0200, Daniel Borkmann wrote:
> On 07/27/2018 11:52 PM, Roman Gushchin wrote:
> [...]
> > @@ -2533,6 +2541,16 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> >  	}
> >  
> >  	regs = cur_regs(env);
> > +
> > +	/* check that flags argument in get_local_storage(map, flags) is 0,
> > +	 * this is required because get_local_storage() can't return an error.
> > +	 */
> > +	if (func_id == BPF_FUNC_get_local_storage &&
> > +	    !tnum_equals_const(regs[BPF_REG_2].var_off, 0)) {
> > +		verbose(env, "get_local_storage() doesn't support non-zero flags\n");
> > +		return -EINVAL;
> > +	}
> 
> Hmm, this check is actually not correct. You will still be able to pass non-zero
> values in there. arg2_type from the helper is ARG_ANYTHING, so the register type
> could for example be one of the pointer types and it will still pass the verifier.
> The correct way to check would be to use register_is_null().
> 
> > +
> >  	/* reset caller saved regs */
> >  	for (i = 0; i < CALLER_SAVED_REGS; i++) {
> >  		mark_reg_not_init(env, regs, caller_saved[i]);

Oh, perfect catch!
The diff is below. Please, let me know if you prefer me to resend
the whole patch/patchset.

Thank you!

Roman

--

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0f69f827ff9f..4547e07ab3db 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2546,7 +2546,7 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
         * this is required because get_local_storage() can't return an error.
         */
        if (func_id == BPF_FUNC_get_local_storage &&
-           !tnum_equals_const(regs[BPF_REG_2].var_off, 0)) {
+           !register_is_null(&regs[BPF_REG_2])) {
                verbose(env, "get_local_storage() doesn't support non-zero flags\n");
                return -EINVAL;
        }