bpf: Fix a bpf_kptr_xchg() issue with local kptr

When reviewing local percpu kptr support, Alexei discovered a bug
wherea bpf_kptr_xchg() may succeed even if the map value kptr type and
locally allocated obj type do not match ([1]). Missed struct btf_id
comparison is the reason for the bug. This patch added such struct btf_id
comparison and will flag verification failure if types do not match.

  [1] https://lore.kernel.org/bpf/20230819002907.io3iphmnuk43xblu@macbook-pro-8.dhcp.thefacebook.com/#t

Reported-by: Alexei Starovoitov <ast@kernel.org>
Fixes: 738c96d ("bpf: Allow local kptrs to be exchanged via bpf_kptr_xchg")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230822050053.2886960-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Yonghong Song

kernel/bpf/verifier.c

@@ -4990,20 +4990,22 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
 			       struct bpf_reg_state *reg, u32 regno)
 {
 	const char *targ_name = btf_type_name(kptr_field->kptr.btf, kptr_field->kptr.btf_id);
-	int perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED | MEM_RCU;
+	int perm_flags;
 	const char *reg_name = "";
 
-	/* Only unreferenced case accepts untrusted pointers */
-	if (kptr_field->type == BPF_KPTR_UNREF)
-		perm_flags |= PTR_UNTRUSTED;
+	if (btf_is_kernel(reg->btf)) {
+		perm_flags = PTR_MAYBE_NULL | PTR_TRUSTED | MEM_RCU;
+
+		/* Only unreferenced case accepts untrusted pointers */
+		if (kptr_field->type == BPF_KPTR_UNREF)
+			perm_flags |= PTR_UNTRUSTED;
+	} else {
+		perm_flags = PTR_MAYBE_NULL | MEM_ALLOC;
+	}
 
 	if (base_type(reg->type) != PTR_TO_BTF_ID || (type_flag(reg->type) & ~perm_flags))
 		goto bad_type;
 
-	if (!btf_is_kernel(reg->btf)) {
-		verbose(env, "R%d must point to kernel BTF\n", regno);
-		return -EINVAL;
-	}
 	/* We need to verify reg->type and reg->btf, before accessing reg->btf */
 	reg_name = btf_type_name(reg->btf, reg->btf_id);
 
@@ -5016,7 +5018,7 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
 	if (__check_ptr_off_reg(env, reg, regno, true))
 		return -EACCES;
 
-	/* A full type match is needed, as BTF can be vmlinux or module BTF, and
+	/* A full type match is needed, as BTF can be vmlinux, module or prog BTF, and
 	 * we also need to take into account the reg->off.
 	 *
 	 * We want to support cases like:
@@ -7916,7 +7918,10 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 			verbose(env, "verifier internal error: unimplemented handling of MEM_ALLOC\n");
 			return -EFAULT;
 		}
-		/* Handled by helper specific checks */
+		if (meta->func_id == BPF_FUNC_kptr_xchg) {
+			if (map_kptr_match_type(env, meta->kptr_field, reg, regno))
+				return -EACCES;
+		}
 		break;
 	case PTR_TO_BTF_ID | MEM_PERCPU:
 	case PTR_TO_BTF_ID | MEM_PERCPU | PTR_TRUSTED: