bpf: Support bpf_kptr_xchg into local kptr

Currently, users can only stash kptr into map values with bpf_kptr_xchg().
This patch further supports stashing kptr into local kptr by adding local
kptr as a valid destination type.

When stashing into local kptr, btf_record in program BTF is used instead
of btf_record in map to search for the btf_field of the local kptr.

The local kptr specific checks in check_reg_type() only apply when the
source argument of bpf_kptr_xchg() is local kptr. Therefore, we make the
scope of the check explicit as the destination now can also be local kptr.
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Amery Hung <amery.hung@bytedance.com>
Link: https://lore.kernel.org/r/20240813212424.2871455-5-amery.hung@bytedance.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -5519,11 +5519,12 @@ union bpf_attr {
  *		**-EOPNOTSUPP** if the hash calculation failed or **-EINVAL** if
  *		invalid arguments are passed.
  *
- * void *bpf_kptr_xchg(void *map_value, void *ptr)
+ * void *bpf_kptr_xchg(void *dst, void *ptr)
  *	Description
- *		Exchange kptr at pointer *map_value* with *ptr*, and return the
- *		old value. *ptr* can be NULL, otherwise it must be a referenced
- *		pointer which will be released when this helper is called.
+ *		Exchange kptr at pointer *dst* with *ptr*, and return the old value.
+ *		*dst* can be map value or local kptr. *ptr* can be NULL, otherwise
+ *		it must be a referenced pointer which will be released when this helper
+ *		is called.
  *	Return
  *		The old value of kptr (which can be NULL). The returned pointer
  *		if not NULL, is a reference which must be released using its

kernel/bpf/helpers.c

@@ -1619,9 +1619,9 @@ void bpf_wq_cancel_and_free(void *val)
 	schedule_work(&work->delete_work);
 }
 
-BPF_CALL_2(bpf_kptr_xchg, void *, map_value, void *, ptr)
+BPF_CALL_2(bpf_kptr_xchg, void *, dst, void *, ptr)
 {
-	unsigned long *kptr = map_value;
+	unsigned long *kptr = dst;
 
 	/* This helper may be inlined by verifier. */
 	return xchg(kptr, (unsigned long)ptr);

kernel/bpf/verifier.c

@@ -7803,29 +7803,38 @@ static int process_kptr_func(struct bpf_verifier_env *env, int regno,
 			     struct bpf_call_arg_meta *meta)
 {
 	struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
-	struct bpf_map *map_ptr = reg->map_ptr;
 	struct btf_field *kptr_field;
+	struct bpf_map *map_ptr;
+	struct btf_record *rec;
 	u32 kptr_off;
 
+	if (type_is_ptr_alloc_obj(reg->type)) {
+		rec = reg_btf_record(reg);
+	} else { /* PTR_TO_MAP_VALUE */
+		map_ptr = reg->map_ptr;
+		if (!map_ptr->btf) {
+			verbose(env, "map '%s' has to have BTF in order to use bpf_kptr_xchg\n",
+				map_ptr->name);
+			return -EINVAL;
+		}
+		rec = map_ptr->record;
+		meta->map_ptr = map_ptr;
+	}
+
 	if (!tnum_is_const(reg->var_off)) {
 		verbose(env,
 			"R%d doesn't have constant offset. kptr has to be at the constant offset\n",
 			regno);
 		return -EINVAL;
 	}
-	if (!map_ptr->btf) {
-		verbose(env, "map '%s' has to have BTF in order to use bpf_kptr_xchg\n",
-			map_ptr->name);
-		return -EINVAL;
-	}
-	if (!btf_record_has_field(map_ptr->record, BPF_KPTR)) {
-		verbose(env, "map '%s' has no valid kptr\n", map_ptr->name);
+
+	if (!btf_record_has_field(rec, BPF_KPTR)) {
+		verbose(env, "R%d has no valid kptr\n", regno);
 		return -EINVAL;
 	}
 
-	meta->map_ptr = map_ptr;
 	kptr_off = reg->off + reg->var_off.value;
-	kptr_field = btf_record_find(map_ptr->record, kptr_off, BPF_KPTR);
+	kptr_field = btf_record_find(rec, kptr_off, BPF_KPTR);
 	if (!kptr_field) {
 		verbose(env, "off=%d doesn't point to kptr\n", kptr_off);
 		return -EACCES;
@@ -8412,7 +8421,12 @@ static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
 static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
 static const struct bpf_reg_types const_str_ptr_types = { .types = { PTR_TO_MAP_VALUE } };
 static const struct bpf_reg_types timer_types = { .types = { PTR_TO_MAP_VALUE } };
-static const struct bpf_reg_types kptr_xchg_dest_types = { .types = { PTR_TO_MAP_VALUE } };
+static const struct bpf_reg_types kptr_xchg_dest_types = {
+	.types = {
+		PTR_TO_MAP_VALUE,
+		PTR_TO_BTF_ID | MEM_ALLOC
+	}
+};
 static const struct bpf_reg_types dynptr_types = {
 	.types = {
 		PTR_TO_STACK,
@@ -8483,7 +8497,8 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 	if (base_type(arg_type) == ARG_PTR_TO_MEM)
 		type &= ~DYNPTR_TYPE_FLAG_MASK;
 
-	if (meta->func_id == BPF_FUNC_kptr_xchg && type_is_alloc(type)) {
+	/* Local kptr types are allowed as the source argument of bpf_kptr_xchg */
+	if (meta->func_id == BPF_FUNC_kptr_xchg && type_is_alloc(type) && regno == BPF_REG_2) {
 		type &= ~MEM_ALLOC;
 		type &= ~MEM_PERCPU;
 	}
@@ -8576,7 +8591,8 @@ 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;
 		}
-		if (meta->func_id == BPF_FUNC_kptr_xchg) {
+		/* Check if local kptr in src arg matches kptr in dst arg */
+		if (meta->func_id == BPF_FUNC_kptr_xchg && regno == BPF_REG_2) {
 			if (map_kptr_match_type(env, meta->kptr_field, reg, regno))
 				return -EACCES;
 		}
@@ -8887,7 +8903,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		meta->release_regno = regno;
 	}
 
-	if (reg->ref_obj_id) {
+	if (reg->ref_obj_id && base_type(arg_type) != ARG_KPTR_XCHG_DEST) {
 		if (meta->ref_obj_id) {
 			verbose(env, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
 				regno, reg->ref_obj_id,