Re: [PATCH bpf-next 3/8] bpf: Disallow NULL PTR_TO_MEM for trusted kfuncs

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Fri, Jan 20, 2023 at 05:28:28AM IST, David Vernet wrote:
> KF_TRUSTED_ARGS kfuncs currently have a subtle and insidious bug in
> validating pointers to scalars. Say that you have a kfunc like the
> following, which takes an array as the first argument:
>
> bool bpf_cpumask_empty(const struct cpumask *cpumask)
> {
> 	return cpumask_empty(cpumask);
> }
>
> ...
> BTF_ID_FLAGS(func, bpf_cpumask_empty, KF_TRUSTED_ARGS)
> ...
>

This is known and expected.

> If a BPF program were to invoke the kfunc with a NULL argument, it would
> crash the kernel. The reason is that struct cpumask is defined as a
> bitmap, which is itself defined as an array, and is accessed as a memory
> address memory by bitmap operations. So when the verifier analyzes the
> register, it interprets it as a pointer to a scalar struct, which is an
> array of size 8. check_mem_reg() then sees that the register is NULL,
> and returns 0, and the kfunc crashes when it passes it down to the
> cpumask wrappers.
>
> To fix this, this patch adds a check for KF_ARG_PTR_TO_MEM which
> verifies that the register doesn't contain a NULL pointer if the kfunc
> is KF_TRUSTED_ARGS.
>
> This may or may not be desired behavior. Some kfuncs may want to
> allow callers to pass NULL-able pointers. An alternative would be adding
> a KF_NOT_NULL flag and leaving KF_TRUSTED_ARGS alone, though given that
> a kfunc is saying it wants to "trust" an argument, it seems reasonable
> to prevent NULL.
>
> Signed-off-by: David Vernet <void@manifault.com>
> ---
>  kernel/bpf/verifier.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9fa101420046..28ccb92ebe65 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9092,6 +9092,11 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
>  					i, btf_type_str(ref_t), ref_tname, PTR_ERR(resolve_ret));
>  				return -EINVAL;
>  			}
> +			if (is_kfunc_trusted_args(meta) && register_is_null(reg)) {
> +				verbose(env, "NULL pointer passed to trusted arg%d\n", i);
> +				return -EACCES;
> +			}
> +

Current patch looks like a stop gap solution. Just checking for register_is_null
is not enough, what about PTR_MAYBE_NULL? That can also be passed. Some
arguments can be both PTR_TO_BTF_ID and PTR_TO_MEM, so it will be bypassed in
the other case because this check is limited to KF_ARG_PTR_TO_MEM. It would
probably be better to disallow NULL by default and explicitly tag the argument
with __or_null to indicate that NULL is accepted. Seems like a much better
default to me.