bpf: Add reference tracking support to kfunc

This patch adds verifier support for PTR_TO_BTF_ID return type of kfunc
to be a reference, by reusing acquire_reference_state/release_reference
support for existing in-kernel bpf helpers.

We make use of the three kfunc types:

- BTF_KFUNC_TYPE_ACQUIRE
  Return true if kfunc_btf_id is an acquire kfunc.  This will
  acquire_reference_state for the returned PTR_TO_BTF_ID (this is the
  only allow return value). Note that acquire kfunc must always return a
  PTR_TO_BTF_ID{_OR_NULL}, otherwise the program is rejected.

- BTF_KFUNC_TYPE_RELEASE
  Return true if kfunc_btf_id is a release kfunc.  This will release the
  reference to the passed in PTR_TO_BTF_ID which has a reference state
  (from earlier acquire kfunc).
  The btf_check_func_arg_match returns the regno (of argument register,
  hence > 0) if the kfunc is a release kfunc, and a proper referenced
  PTR_TO_BTF_ID is being passed to it.
  This is similar to how helper call check uses bpf_call_arg_meta to
  store the ref_obj_id that is later used to release the reference.
  Similar to in-kernel helper, we only allow passing one referenced
  PTR_TO_BTF_ID as an argument. It can also be passed in to normal
  kfunc, but in case of release kfunc there must always be one
  PTR_TO_BTF_ID argument that is referenced.

- BTF_KFUNC_TYPE_RET_NULL
  For kfunc returning PTR_TO_BTF_ID, tells if it can be NULL, hence
  force caller to mark the pointer not null (using check) before
  accessing it. Note that taking into account the case fixed by commit
  93c230e ("bpf: Enforce id generation for all may-be-null register type")
  we assign a non-zero id for mark_ptr_or_null_reg logic. Later, if more
  return types are supported by kfunc, which have a _OR_NULL variant, it
  might be better to move this id generation under a common
  reg_type_may_be_null check, similar to the case in the commit.

Referenced PTR_TO_BTF_ID is currently only limited to kfunc, but can be
extended in the future to other BPF helpers as well.  For now, we can
rely on the btf_struct_ids_match check to ensure we get the pointer to
the expected struct type. In the future, care needs to be taken to avoid
ambiguity for reference PTR_TO_BTF_ID passed to release function, in
case multiple candidates can release same BTF ID.

e.g. there might be two release kfuncs (or kfunc and helper):

foo(struct abc *p);
bar(struct abc *p);

... such that both release a PTR_TO_BTF_ID with btf_id of struct abc. In
this case we would need to track the acquire function corresponding to
the release function to avoid type confusion, and store this information
in the register state so that an incorrect program can be rejected. This
is not a problem right now, hence it is left as an exercise for the
future patch introducing such a case in the kernel.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220114163953.1455836-6-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf_verifier.h

@@ -566,4 +566,9 @@ static inline u32 type_flag(u32 type)
 	return type & ~BPF_BASE_TYPE_MASK;
 }
 
+static inline enum bpf_prog_type resolve_prog_type(struct bpf_prog *prog)
+{
+	return prog->aux->dst_prog ? prog->aux->dst_prog->type : prog->type;
+}
+
 #endif /* _LINUX_BPF_VERIFIER_H */

kernel/bpf/btf.c

@@ -5686,11 +5686,13 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 				    bool ptr_to_mem_ok)
 {
 	struct bpf_verifier_log *log = &env->log;
+	u32 i, nargs, ref_id, ref_obj_id = 0;
 	bool is_kfunc = btf_is_kernel(btf);
 	const char *func_name, *ref_tname;
 	const struct btf_type *t, *ref_t;
 	const struct btf_param *args;
-	u32 i, nargs, ref_id;
+	int ref_regno = 0;
+	bool rel = false;
 
 	t = btf_type_by_id(btf, func_id);
 	if (!t || !btf_type_is_func(t)) {
@@ -5768,6 +5770,16 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 			if (reg->type == PTR_TO_BTF_ID) {
 				reg_btf = reg->btf;
 				reg_ref_id = reg->btf_id;
+				/* Ensure only one argument is referenced PTR_TO_BTF_ID */
+				if (reg->ref_obj_id) {
+					if (ref_obj_id) {
+						bpf_log(log, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
+							regno, reg->ref_obj_id, ref_obj_id);
+						return -EFAULT;
+					}
+					ref_regno = regno;
+					ref_obj_id = reg->ref_obj_id;
+				}
 			} else {
 				reg_btf = btf_vmlinux;
 				reg_ref_id = *reg2btf_ids[reg->type];
@@ -5838,7 +5850,23 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 		}
 	}
 
-	return 0;
+	/* Either both are set, or neither */
+	WARN_ON_ONCE((ref_obj_id && !ref_regno) || (!ref_obj_id && ref_regno));
+	if (is_kfunc) {
+		rel = btf_kfunc_id_set_contains(btf, resolve_prog_type(env->prog),
+						BTF_KFUNC_TYPE_RELEASE, func_id);
+		/* We already made sure ref_obj_id is set only for one argument */
+		if (rel && !ref_obj_id) {
+			bpf_log(log, "release kernel function %s expects refcounted PTR_TO_BTF_ID\n",
+				func_name);
+			return -EINVAL;
+		}
+		/* Allow (!rel && ref_obj_id), so that passing such referenced PTR_TO_BTF_ID to
+		 * other kfuncs works
+		 */
+	}
+	/* returns argument register number > 0 in case of reference release kfunc */
+	return rel ? ref_regno : 0;
 }
 
 /* Compare BTF of a function with given bpf_reg_state.

kernel/bpf/verifier.c

@@ -452,7 +452,8 @@ static bool reg_type_may_be_refcounted_or_null(enum bpf_reg_type type)
 {
 	return base_type(type) == PTR_TO_SOCKET ||
 		base_type(type) == PTR_TO_TCP_SOCK ||
-		base_type(type) == PTR_TO_MEM;
+		base_type(type) == PTR_TO_MEM ||
+		base_type(type) == PTR_TO_BTF_ID;
 }
 
 static bool type_is_rdonly_mem(u32 type)
@@ -3493,11 +3494,6 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno,
 
 #define MAX_PACKET_OFF 0xffff
 
-static enum bpf_prog_type resolve_prog_type(struct bpf_prog *prog)
-{
-	return prog->aux->dst_prog ? prog->aux->dst_prog->type : prog->type;
-}
-
 static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
 				       const struct bpf_call_arg_meta *meta,
 				       enum bpf_access_type t)
@@ -6845,15 +6841,17 @@ static void mark_btf_func_reg_size(struct bpf_verifier_env *env, u32 regno,
 	}
 }
 
-static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn)
+static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
+			    int *insn_idx_p)
 {
 	const struct btf_type *t, *func, *func_proto, *ptr_type;
 	struct bpf_reg_state *regs = cur_regs(env);
 	const char *func_name, *ptr_type_name;
 	u32 i, nargs, func_id, ptr_type_id;
+	int err, insn_idx = *insn_idx_p;
 	const struct btf_param *args;
 	struct btf *desc_btf;
-	int err;
+	bool acq;
 
 	/* skip for now, but return error when we find this in fixup_kfunc_call */
 	if (!insn->imm)
@@ -6875,16 +6873,36 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn)
 		return -EACCES;
 	}
 
+	acq = btf_kfunc_id_set_contains(desc_btf, resolve_prog_type(env->prog),
+					BTF_KFUNC_TYPE_ACQUIRE, func_id);
+
 	/* Check the arguments */
 	err = btf_check_kfunc_arg_match(env, desc_btf, func_id, regs);
-	if (err)
+	if (err < 0)
 		return err;
+	/* In case of release function, we get register number of refcounted
+	 * PTR_TO_BTF_ID back from btf_check_kfunc_arg_match, do the release now
+	 */
+	if (err) {
+		err = release_reference(env, regs[err].ref_obj_id);
+		if (err) {
+			verbose(env, "kfunc %s#%d reference has not been acquired before\n",
+				func_name, func_id);
+			return err;
+		}
+	}
 
 	for (i = 0; i < CALLER_SAVED_REGS; i++)
 		mark_reg_not_init(env, regs, caller_saved[i]);
 
 	/* Check return type */
 	t = btf_type_skip_modifiers(desc_btf, func_proto->type, NULL);
+
+	if (acq && !btf_type_is_ptr(t)) {
+		verbose(env, "acquire kernel function does not return PTR_TO_BTF_ID\n");
+		return -EINVAL;
+	}
+
 	if (btf_type_is_scalar(t)) {
 		mark_reg_unknown(env, regs, BPF_REG_0);
 		mark_btf_func_reg_size(env, BPF_REG_0, t->size);
@@ -6903,7 +6921,21 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn)
 		regs[BPF_REG_0].btf = desc_btf;
 		regs[BPF_REG_0].type = PTR_TO_BTF_ID;
 		regs[BPF_REG_0].btf_id = ptr_type_id;
+		if (btf_kfunc_id_set_contains(desc_btf, resolve_prog_type(env->prog),
+					      BTF_KFUNC_TYPE_RET_NULL, func_id)) {
+			regs[BPF_REG_0].type |= PTR_MAYBE_NULL;
+			/* For mark_ptr_or_null_reg, see 93c230e3f5bd6 */
+			regs[BPF_REG_0].id = ++env->id_gen;
+		}
 		mark_btf_func_reg_size(env, BPF_REG_0, sizeof(void *));
+		if (acq) {
+			int id = acquire_reference_state(env, insn_idx);
+
+			if (id < 0)
+				return id;
+			regs[BPF_REG_0].id = id;
+			regs[BPF_REG_0].ref_obj_id = id;
+		}
 	} /* else { add_kfunc_call() ensures it is btf_type_is_void(t) } */
 
 	nargs = btf_type_vlen(func_proto);
@@ -11548,7 +11580,7 @@ static int do_check(struct bpf_verifier_env *env)
 				if (insn->src_reg == BPF_PSEUDO_CALL)
 					err = check_func_call(env, insn, &env->insn_idx);
 				else if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL)
-					err = check_kfunc_call(env, insn);
+					err = check_kfunc_call(env, insn, &env->insn_idx);
 				else
 					err = check_helper_call(env, insn, &env->insn_idx);
 				if (err)