Re: [PATCH bpf 1/2] bpf: Fix calling global functions from BPF_PROG_TYPE_EXT programs

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

On Fri, Jun 03, 2022 at 09:10:26PM IST, Toke Høiland-Jørgensen wrote:
> The verifier allows programs to call global functions as long as their
> argument types match, using BTF to check the function arguments. One of the
> allowed argument types to such global functions is PTR_TO_CTX; however the
> check for this fails on BPF_PROG_TYPE_EXT functions because the verifier
> uses the wrong type to fetch the vmlinux BTF ID for the program context
> type. This failure is seen when an XDP program is loaded using
> libxdp (which loads it as BPF_PROG_TYPE_EXT and attaches it to a global XDP
> type program).
>
> Fix the issue by passing in the target program type instead of the
> BPF_PROG_TYPE_EXT type to bpf_prog_get_ctx() when checking function
> argument compatibility.
>
> The first Fixes tag refers to the latest commit that touched the code in
> question, while the second one points to the code that first introduced
> the global function call verification.
>
> Fixes: 3363bd0cfbb8 ("bpf: Extend kfunc with PTR_TO_CTX, PTR_TO_MEM argument support")
> Fixes: 51c39bb1d5d1 ("bpf: Introduce function-by-function verification")
> Reported-by: Simon Sundberg <simon.sundberg@kau.se>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>  kernel/bpf/btf.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 7bccaa4646e5..361de7304c4d 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6054,6 +6054,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>  				    struct bpf_reg_state *regs,
>  				    bool ptr_to_mem_ok)
>  {
> +	enum bpf_prog_type prog_type = env->prog->type;
>  	struct bpf_verifier_log *log = &env->log;
>  	u32 i, nargs, ref_id, ref_obj_id = 0;
>  	bool is_kfunc = btf_is_kernel(btf);
> @@ -6095,6 +6096,9 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>  						     BTF_KFUNC_TYPE_KPTR_ACQUIRE, func_id);
>  	}
>
> +	if (prog_type == BPF_PROG_TYPE_EXT && env->prog->aux->dst_prog)
> +		prog_type = env->prog->aux->dst_prog->type;
> +

nit: it might be better to reuse resolve_prog_type here.

>  	/* check that BTF function arguments match actual types that the
>  	 * verifier sees.
>  	 */
> @@ -6171,7 +6175,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
>  				return -EINVAL;
>  			}
>  			/* rest of the arguments can be anything, like normal kfunc */
> -		} else if (btf_get_prog_ctx_type(log, btf, t, env->prog->type, i)) {
> +		} else if (btf_get_prog_ctx_type(log, btf, t, prog_type, i)) {
>  			/* If function expects ctx type in BTF check that caller
>  			 * is passing PTR_TO_CTX.
>  			 */
> --
> 2.36.1
>

--
Kartikeya