bpf: Add kfunc bpf_rcu_read_lock/unlock()

Add two kfunc's bpf_rcu_read_lock() and bpf_rcu_read_unlock(). These two kfunc's
can be used for all program types. The following is an example about how
rcu pointer are used w.r.t. bpf_rcu_read_lock()/bpf_rcu_read_unlock().

  struct task_struct {
    ...
    struct task_struct              *last_wakee;
    struct task_struct __rcu        *real_parent;
    ...
  };

Let us say prog does 'task = bpf_get_current_task_btf()' to get a
'task' pointer. The basic rules are:
  - 'real_parent = task->real_parent' should be inside bpf_rcu_read_lock
    region. This is to simulate rcu_dereference() operation. The
    'real_parent' is marked as MEM_RCU only if (1). task->real_parent is
    inside bpf_rcu_read_lock region, and (2). task is a trusted ptr. So
    MEM_RCU marked ptr can be 'trusted' inside the bpf_rcu_read_lock region.
  - 'last_wakee = real_parent->last_wakee' should be inside bpf_rcu_read_lock
    region since it tries to access rcu protected memory.
  - the ptr 'last_wakee' will be marked as PTR_UNTRUSTED since in general
    it is not clear whether the object pointed by 'last_wakee' is valid or
    not even inside bpf_rcu_read_lock region.

The verifier will reset all rcu pointer register states to untrusted
at bpf_rcu_read_unlock() kfunc call site, so any such rcu pointer
won't be trusted any more outside the bpf_rcu_read_lock() region.

The current implementation does not support nested rcu read lock
region in the prog.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221124053217.2373910-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -572,6 +572,9 @@ enum bpf_type_flag {
 	 */
 	PTR_TRUSTED		= BIT(12 + BPF_BASE_TYPE_BITS),
 
+	/* MEM is tagged with rcu and memory access needs rcu_read_lock protection. */
+	MEM_RCU			= BIT(13 + BPF_BASE_TYPE_BITS),
+
 	__BPF_TYPE_FLAG_MAX,
 	__BPF_TYPE_LAST_FLAG	= __BPF_TYPE_FLAG_MAX - 1,
 };

include/linux/bpf_verifier.h

@@ -344,6 +344,7 @@ struct bpf_verifier_state {
 		u32 id;
 	} active_lock;
 	bool speculative;
+	bool active_rcu_lock;
 
 	/* first and last insn idx of this verifier state */
 	u32 first_insn_idx;
@@ -445,6 +446,7 @@ struct bpf_insn_aux_data {
 	u32 seen; /* this insn was processed by the verifier at env->pass_cnt */
 	bool sanitize_stack_spill; /* subject to Spectre v4 sanitation */
 	bool zext_dst; /* this insn zero extends dst reg */
+	bool storage_get_func_atomic; /* bpf_*_storage_get() with atomic memory alloc */
 	u8 alu_state; /* used in combination with alu_limit */
 
 	/* below fields are initialized once */
@@ -534,6 +536,7 @@ struct bpf_verifier_env {
 	bool bypass_spec_v1;
 	bool bypass_spec_v4;
 	bool seen_direct_write;
+	bool rcu_tag_supported;
 	struct bpf_insn_aux_data *insn_aux_data; /* array of per-insn state */
 	const struct bpf_line_info *prev_linfo;
 	struct bpf_verifier_log log;
@@ -680,7 +683,7 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
 	}
 }
 
-#define BPF_REG_TRUSTED_MODIFIERS (MEM_ALLOC | PTR_TRUSTED)
+#define BPF_REG_TRUSTED_MODIFIERS (MEM_ALLOC | MEM_RCU | PTR_TRUSTED)
 
 static inline bool bpf_type_has_unsafe_modifiers(u32 type)
 {

kernel/bpf/btf.c

@@ -6238,6 +6238,9 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 				/* check __percpu tag */
 				if (strcmp(tag_value, "percpu") == 0)
 					tmp_flag = MEM_PERCPU;
+				/* check __rcu tag */
+				if (strcmp(tag_value, "rcu") == 0)
+					tmp_flag = MEM_RCU;
 			}
 
 			stype = btf_type_skip_modifiers(btf, mtype->type, &id);

kernel/bpf/helpers.c

@@ -1990,6 +1990,16 @@ void *bpf_rdonly_cast(void *obj__ign, u32 btf_id__k)
 	return obj__ign;
 }
 
+void bpf_rcu_read_lock(void)
+{
+	rcu_read_lock();
+}
+
+void bpf_rcu_read_unlock(void)
+{
+	rcu_read_unlock();
+}
+
 __diag_pop();
 
 BTF_SET8_START(generic_btf_ids)
@@ -2031,6 +2041,8 @@ BTF_ID(func, bpf_cgroup_release)
 BTF_SET8_START(common_btf_ids)
 BTF_ID_FLAGS(func, bpf_cast_to_kern_ctx)
 BTF_ID_FLAGS(func, bpf_rdonly_cast)
+BTF_ID_FLAGS(func, bpf_rcu_read_lock)
+BTF_ID_FLAGS(func, bpf_rcu_read_unlock)
 BTF_SET8_END(common_btf_ids)
 
 static const struct btf_kfunc_id_set common_kfunc_set = {

kernel/bpf/verifier.c

@@ -527,6 +527,14 @@ static bool is_callback_calling_function(enum bpf_func_id func_id)
 	       func_id == BPF_FUNC_user_ringbuf_drain;
 }
 
+static bool is_storage_get_function(enum bpf_func_id func_id)
+{
+	return func_id == BPF_FUNC_sk_storage_get ||
+	       func_id == BPF_FUNC_inode_storage_get ||
+	       func_id == BPF_FUNC_task_storage_get ||
+	       func_id == BPF_FUNC_cgrp_storage_get;
+}
+
 static bool helper_multiple_ref_obj_use(enum bpf_func_id func_id,
 					const struct bpf_map *map)
 {
@@ -589,11 +597,12 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
 			strncpy(postfix, "_or_null", 16);
 	}
 
-	snprintf(prefix, sizeof(prefix), "%s%s%s%s%s%s",
+	snprintf(prefix, sizeof(prefix), "%s%s%s%s%s%s%s",
 		 type & MEM_RDONLY ? "rdonly_" : "",
 		 type & MEM_RINGBUF ? "ringbuf_" : "",
 		 type & MEM_USER ? "user_" : "",
 		 type & MEM_PERCPU ? "percpu_" : "",
+		 type & MEM_RCU ? "rcu_" : "",
 		 type & PTR_UNTRUSTED ? "untrusted_" : "",
 		 type & PTR_TRUSTED ? "trusted_" : ""
 	);
@@ -1220,6 +1229,7 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,
 		dst_state->frame[i] = NULL;
 	}
 	dst_state->speculative = src->speculative;
+	dst_state->active_rcu_lock = src->active_rcu_lock;
 	dst_state->curframe = src->curframe;
 	dst_state->active_lock.ptr = src->active_lock.ptr;
 	dst_state->active_lock.id = src->active_lock.id;
@@ -4258,6 +4268,25 @@ static bool is_flow_key_reg(struct bpf_verifier_env *env, int regno)
 	return reg->type == PTR_TO_FLOW_KEYS;
 }
 
+static bool is_trusted_reg(const struct bpf_reg_state *reg)
+{
+	/* A referenced register is always trusted. */
+	if (reg->ref_obj_id)
+		return true;
+
+	/* If a register is not referenced, it is trusted if it has the
+	 * MEM_ALLOC, MEM_RCU or PTR_TRUSTED type modifiers, and no others. Some of the
+	 * other type modifiers may be safe, but we elect to take an opt-in
+	 * approach here as some (e.g. PTR_UNTRUSTED and PTR_MAYBE_NULL) are
+	 * not.
+	 *
+	 * Eventually, we should make PTR_TRUSTED the single source of truth
+	 * for whether a register is trusted.
+	 */
+	return type_flag(reg->type) & BPF_REG_TRUSTED_MODIFIERS &&
+	       !bpf_type_has_unsafe_modifiers(reg->type);
+}
+
 static int check_pkt_ptr_alignment(struct bpf_verifier_env *env,
 				   const struct bpf_reg_state *reg,
 				   int off, int size, bool strict)
@@ -4737,9 +4766,28 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 	if (type_flag(reg->type) & PTR_UNTRUSTED)
 		flag |= PTR_UNTRUSTED;
 
-	/* Any pointer obtained from walking a trusted pointer is no longer trusted. */
+	/* By default any pointer obtained from walking a trusted pointer is
+	 * no longer trusted except the rcu case below.
+	 */
 	flag &= ~PTR_TRUSTED;
 
+	if (flag & MEM_RCU) {
+		/* Mark value register as MEM_RCU only if it is protected by
+		 * bpf_rcu_read_lock() and the ptr reg is trusted. MEM_RCU
+		 * itself can already indicate trustedness inside the rcu
+		 * read lock region. Also mark it as PTR_TRUSTED.
+		 */
+		if (!env->cur_state->active_rcu_lock || !is_trusted_reg(reg))
+			flag &= ~MEM_RCU;
+		else
+			flag |= PTR_TRUSTED;
+	} else if (reg->type & MEM_RCU) {
+		/* ptr (reg) is marked as MEM_RCU, but the struct field is not tagged
+		 * with __rcu. Mark the flag as PTR_UNTRUSTED conservatively.
+		 */
+		flag |= PTR_UNTRUSTED;
+	}
+
 	if (atype == BPF_READ && value_regno >= 0)
 		mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id, flag);
 
@@ -5897,6 +5945,7 @@ static const struct bpf_reg_types btf_ptr_types = {
 	.types = {
 		PTR_TO_BTF_ID,
 		PTR_TO_BTF_ID | PTR_TRUSTED,
+		PTR_TO_BTF_ID | MEM_RCU | PTR_TRUSTED,
 	},
 };
 static const struct bpf_reg_types percpu_btf_ptr_types = {
@@ -6075,6 +6124,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	case PTR_TO_BTF_ID:
 	case PTR_TO_BTF_ID | MEM_ALLOC:
 	case PTR_TO_BTF_ID | PTR_TRUSTED:
+	case PTR_TO_BTF_ID | MEM_RCU | PTR_TRUSTED:
 	case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
 		/* When referenced PTR_TO_BTF_ID is passed to release function,
 		 * it's fixed offset must be 0.	In the other cases, fixed offset
@@ -7539,6 +7589,17 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 		return err;
 	}
 
+	if (env->cur_state->active_rcu_lock) {
+		if (fn->might_sleep) {
+			verbose(env, "sleepable helper %s#%d in rcu_read_lock region\n",
+				func_id_name(func_id), func_id);
+			return -EINVAL;
+		}
+
+		if (env->prog->aux->sleepable && is_storage_get_function(func_id))
+			env->insn_aux_data[insn_idx].storage_get_func_atomic = true;
+	}
+
 	meta.func_id = func_id;
 	/* check args */
 	for (i = 0; i < MAX_BPF_FUNC_REG_ARGS; i++) {
@@ -7966,25 +8027,6 @@ static bool is_kfunc_arg_kptr_get(struct bpf_kfunc_call_arg_meta *meta, int arg)
 	return arg == 0 && (meta->kfunc_flags & KF_KPTR_GET);
 }
 
-static bool is_trusted_reg(const struct bpf_reg_state *reg)
-{
-	/* A referenced register is always trusted. */
-	if (reg->ref_obj_id)
-		return true;
-
-	/* If a register is not referenced, it is trusted if it has either the
-	 * MEM_ALLOC or PTR_TRUSTED type modifiers, and no others. Some of the
-	 * other type modifiers may be safe, but we elect to take an opt-in
-	 * approach here as some (e.g. PTR_UNTRUSTED and PTR_MAYBE_NULL) are
-	 * not.
-	 *
-	 * Eventually, we should make PTR_TRUSTED the single source of truth
-	 * for whether a register is trusted.
-	 */
-	return type_flag(reg->type) & BPF_REG_TRUSTED_MODIFIERS &&
-	       !bpf_type_has_unsafe_modifiers(reg->type);
-}
-
 static bool __kfunc_param_match_suffix(const struct btf *btf,
 				       const struct btf_param *arg,
 				       const char *suffix)
@@ -8163,6 +8205,8 @@ enum special_kfunc_type {
 	KF_bpf_list_pop_back,
 	KF_bpf_cast_to_kern_ctx,
 	KF_bpf_rdonly_cast,
+	KF_bpf_rcu_read_lock,
+	KF_bpf_rcu_read_unlock,
 };
 
 BTF_SET_START(special_kfunc_set)
@@ -8185,6 +8229,18 @@ BTF_ID(func, bpf_list_pop_front)
 BTF_ID(func, bpf_list_pop_back)
 BTF_ID(func, bpf_cast_to_kern_ctx)
 BTF_ID(func, bpf_rdonly_cast)
+BTF_ID(func, bpf_rcu_read_lock)
+BTF_ID(func, bpf_rcu_read_unlock)
+
+static bool is_kfunc_bpf_rcu_read_lock(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return meta->func_id == special_kfunc_list[KF_bpf_rcu_read_lock];
+}
+
+static bool is_kfunc_bpf_rcu_read_unlock(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return meta->func_id == special_kfunc_list[KF_bpf_rcu_read_unlock];
+}
 
 static enum kfunc_ptr_arg_type
 get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
@@ -8817,6 +8873,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 	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;
+	bool sleepable, rcu_lock, rcu_unlock;
 	struct bpf_kfunc_call_arg_meta meta;
 	u32 i, nargs, func_id, ptr_type_id;
 	int err, insn_idx = *insn_idx_p;
@@ -8858,11 +8915,45 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		return -EACCES;
 	}
 
-	if (is_kfunc_sleepable(&meta) && !env->prog->aux->sleepable) {
+	sleepable = is_kfunc_sleepable(&meta);
+	if (sleepable && !env->prog->aux->sleepable) {
 		verbose(env, "program must be sleepable to call sleepable kfunc %s\n", func_name);
 		return -EACCES;
 	}
 
+	rcu_lock = is_kfunc_bpf_rcu_read_lock(&meta);
+	rcu_unlock = is_kfunc_bpf_rcu_read_unlock(&meta);
+	if ((rcu_lock || rcu_unlock) && !env->rcu_tag_supported) {
+		verbose(env, "no vmlinux btf rcu tag support for kfunc %s\n", func_name);
+		return -EACCES;
+	}
+
+	if (env->cur_state->active_rcu_lock) {
+		struct bpf_func_state *state;
+		struct bpf_reg_state *reg;
+
+		if (rcu_lock) {
+			verbose(env, "nested rcu read lock (kernel function %s)\n", func_name);
+			return -EINVAL;
+		} else if (rcu_unlock) {
+			bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
+				if (reg->type & MEM_RCU) {
+					reg->type &= ~(MEM_RCU | PTR_TRUSTED);
+					reg->type |= PTR_UNTRUSTED;
+				}
+			}));
+			env->cur_state->active_rcu_lock = false;
+		} else if (sleepable) {
+			verbose(env, "kernel func %s is sleepable within rcu_read_lock region\n", func_name);
+			return -EACCES;
+		}
+	} else if (rcu_lock) {
+		env->cur_state->active_rcu_lock = true;
+	} else if (rcu_unlock) {
+		verbose(env, "unmatched rcu read unlock (kernel function %s)\n", func_name);
+		return -EINVAL;
+	}
+
 	/* Check the arguments */
 	err = check_kfunc_args(env, &meta);
 	if (err < 0)
@@ -11754,6 +11845,11 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
 		return -EINVAL;
 	}
 
+	if (env->cur_state->active_rcu_lock) {
+		verbose(env, "BPF_LD_[ABS|IND] cannot be used inside bpf_rcu_read_lock-ed region\n");
+		return -EINVAL;
+	}
+
 	if (regs[ctx_reg].type != PTR_TO_CTX) {
 		verbose(env,
 			"at the time of BPF_LD_ABS|IND R6 != pointer to skb\n");
@@ -13019,6 +13115,9 @@ static bool states_equal(struct bpf_verifier_env *env,
 	    old->active_lock.id != cur->active_lock.id)
 		return false;
 
+	if (old->active_rcu_lock != cur->active_rcu_lock)
+		return false;
+
 	/* for states to be equal callsites have to be the same
 	 * and all frame states need to be equivalent
 	 */
@@ -13706,6 +13805,11 @@ static int do_check(struct bpf_verifier_env *env)
 					return -EINVAL;
 				}
 
+				if (env->cur_state->active_rcu_lock) {
+					verbose(env, "bpf_rcu_read_unlock is missing\n");
+					return -EINVAL;
+				}
+
 				/* We must do check_reference_leak here before
 				 * prepare_func_exit to handle the case when
 				 * state->curframe > 0, it may be a callback
@@ -15494,14 +15598,12 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			goto patch_call_imm;
 		}
 
-		if (insn->imm == BPF_FUNC_task_storage_get ||
-		    insn->imm == BPF_FUNC_sk_storage_get ||
-		    insn->imm == BPF_FUNC_inode_storage_get ||
-		    insn->imm == BPF_FUNC_cgrp_storage_get) {
-			if (env->prog->aux->sleepable)
-				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_KERNEL);
-			else
+		if (is_storage_get_function(insn->imm)) {
+			if (!env->prog->aux->sleepable ||
+			    env->insn_aux_data[i + delta].storage_get_func_atomic)
 				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_ATOMIC);
+			else
+				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_5, (__force __s32)GFP_KERNEL);
 			insn_buf[1] = *insn;
 			cnt = 2;
 
@@ -16580,6 +16682,8 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr)
 	env->bypass_spec_v1 = bpf_bypass_spec_v1();
 	env->bypass_spec_v4 = bpf_bypass_spec_v4();
 	env->bpf_capable = bpf_capable();
+	env->rcu_tag_supported = btf_vmlinux &&
+		btf_find_by_name_kind(btf_vmlinux, "rcu", BTF_KIND_TYPE_TAG) > 0;
 
 	if (is_priv)
 		env->test_state_freq = attr->prog_flags & BPF_F_TEST_STATE_FREQ;