bpf: Propagate expected_attach_type when verifying freplace programs

For some program types, the verifier relies on the expected_attach_type of
the program being verified in the verification process. However, for
freplace programs, the attach type was not propagated along with the
verifier ops, so the expected_attach_type would always be zero for freplace
programs.

This in turn caused the verifier to sometimes make the wrong call for
freplace programs. For all existing uses of expected_attach_type for this
purpose, the result of this was only false negatives (i.e., freplace
functions would be rejected by the verifier even though they were valid
programs for the target they were replacing). However, should a false
positive be introduced, this can lead to out-of-bounds accesses and/or
crashes.

The fix introduced in this patch is to propagate the expected_attach_type
to the freplace program during verification, and reset it after that is
done.

Fixes: be8704f ("bpf: Introduce dynamic program extensions")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158773526726.293902.13257293296560360508.stgit@toke.dk

Author: tohojo

kernel/bpf/verifier.c

@@ -10497,6 +10497,7 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 				return -EINVAL;
 			}
 			env->ops = bpf_verifier_ops[tgt_prog->type];
+			prog->expected_attach_type = tgt_prog->expected_attach_type;
 		}
 		if (!tgt_prog->jited) {
 			verbose(env, "Can attach to only JITed progs\n");
@@ -10841,6 +10842,13 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
 		 * them now. Otherwise free_used_maps() will release them.
 		 */
 		release_maps(env);
+
+	/* extension progs temporarily inherit the attach_type of their targets
+	   for verification purposes, so set it back to zero before returning
+	 */
+	if (env->prog->type == BPF_PROG_TYPE_EXT)
+		env->prog->expected_attach_type = 0;
+
 	*prog = env->prog;
 err_unlock:
 	if (!is_priv)