bpf: Fix a potential use-after-free of BTF object

Refcounting in the check_pseudo_btf_id() function is incorrect:
the __check_pseudo_btf_id() function might get called with a zero
refcounted btf. Fix this, and patch related code accordingly.

v3: rephrase a comment (AI)
v2: fix a refcount leak introduced in v1 (AI)

Reported-by: syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5a0f1995634f7c1dadbf
Fixes: 76145f7 ("bpf: Refactor check_pseudo_btf_id")
Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://lore.kernel.org/r/20260209132904.63908-1-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: aspsk

kernel/bpf/verifier.c

@@ -21333,29 +21333,29 @@ static int find_btf_percpu_datasec(struct btf *btf)
 }
 
 /*
- * Add btf to the used_btfs array and return the index. (If the btf was
- * already added, then just return the index.) Upon successful insertion
- * increase btf refcnt, and, if present, also refcount the corresponding
- * kernel module.
+ * Add btf to the env->used_btfs array. If needed, refcount the
+ * corresponding kernel module. To simplify caller's logic
+ * in case of error or if btf was added before the function
+ * decreases the btf refcount.
  */
 static int __add_used_btf(struct bpf_verifier_env *env, struct btf *btf)
 {
 	struct btf_mod_pair *btf_mod;
+	int ret = 0;
 	int i;
 
 	/* check whether we recorded this BTF (and maybe module) already */
 	for (i = 0; i < env->used_btf_cnt; i++)
 		if (env->used_btfs[i].btf == btf)
-			return i;
+			goto ret_put;
 
 	if (env->used_btf_cnt >= MAX_USED_BTFS) {
 		verbose(env, "The total number of btfs per program has reached the limit of %u\n",
 			MAX_USED_BTFS);
-		return -E2BIG;
+		ret = -E2BIG;
+		goto ret_put;
 	}
 
-	btf_get(btf);
-
 	btf_mod = &env->used_btfs[env->used_btf_cnt];
 	btf_mod->btf = btf;
 	btf_mod->module = NULL;
@@ -21364,12 +21364,18 @@ static int __add_used_btf(struct bpf_verifier_env *env, struct btf *btf)
 	if (btf_is_module(btf)) {
 		btf_mod->module = btf_try_get_module(btf);
 		if (!btf_mod->module) {
-			btf_put(btf);
-			return -ENXIO;
+			ret = -ENXIO;
+			goto ret_put;
 		}
 	}
 
-	return env->used_btf_cnt++;
+	env->used_btf_cnt++;
+	return 0;
+
+ret_put:
+	/* Either error or this BTF was already added */
+	btf_put(btf);
+	return ret;
 }
 
 /* replace pseudo btf_id with kernel symbol address */
@@ -21466,9 +21472,7 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 
 	btf_fd = insn[1].imm;
 	if (btf_fd) {
-		CLASS(fd, f)(btf_fd);
-
-		btf = __btf_get_by_fd(f);
+		btf = btf_get_by_fd(btf_fd);
 		if (IS_ERR(btf)) {
 			verbose(env, "invalid module BTF object FD specified.\n");
 			return -EINVAL;
@@ -21478,17 +21482,17 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
 			verbose(env, "kernel is missing BTF, make sure CONFIG_DEBUG_INFO_BTF=y is specified in Kconfig.\n");
 			return -EINVAL;
 		}
+		btf_get(btf_vmlinux);
 		btf = btf_vmlinux;
 	}
 
 	err = __check_pseudo_btf_id(env, insn, aux, btf);
-	if (err)
+	if (err) {
+		btf_put(btf);
 		return err;
+	}
 
-	err = __add_used_btf(env, btf);
-	if (err < 0)
-		return err;
-	return 0;
+	return __add_used_btf(env, btf);
 }
 
 static bool is_tracing_prog_type(enum bpf_prog_type type)
@@ -25368,13 +25372,9 @@ static int add_fd_from_fd_array(struct bpf_verifier_env *env, int fd)
 		return 0;
 	}
 
-	btf = __btf_get_by_fd(f);
-	if (!IS_ERR(btf)) {
-		err = __add_used_btf(env, btf);
-		if (err < 0)
-			return err;
-		return 0;
-	}
+	btf = btf_get_by_fd(fd);
+	if (!IS_ERR(btf))
+		return __add_used_btf(env, btf);
 
 	verbose(env, "fd %d is not pointing to valid bpf_map or btf\n", fd);
 	return PTR_ERR(map);