bpf: Migrate release_on_unlock logic to non-owning ref semantics

This patch introduces non-owning reference semantics to the verifier,
specifically linked_list API kfunc handling. release_on_unlock logic for
refs is refactored - with small functional changes - to implement these
semantics, and bpf_list_push_{front,back} are migrated to use them.

When a list node is pushed to a list, the program still has a pointer to
the node:

  n = bpf_obj_new(typeof(*n));

  bpf_spin_lock(&l);
  bpf_list_push_back(&l, n);
  /* n still points to the just-added node */
  bpf_spin_unlock(&l);

What the verifier considers n to be after the push, and thus what can be
done with n, are changed by this patch.

Common properties both before/after this patch:
  * After push, n is only a valid reference to the node until end of
    critical section
  * After push, n cannot be pushed to any list
  * After push, the program can read the node's fields using n

Before:
  * After push, n retains the ref_obj_id which it received on
    bpf_obj_new, but the associated bpf_reference_state's
    release_on_unlock field is set to true
    * release_on_unlock field and associated logic is used to implement
      "n is only a valid ref until end of critical section"
  * After push, n cannot be written to, the node must be removed from
    the list before writing to its fields
  * After push, n is marked PTR_UNTRUSTED

After:
  * After push, n's ref is released and ref_obj_id set to 0. NON_OWN_REF
    type flag is added to reg's type, indicating that it's a non-owning
    reference.
    * NON_OWN_REF flag and logic is used to implement "n is only a
      valid ref until end of critical section"
  * n can be written to (except for special fields e.g. bpf_list_node,
    timer, ...)

Summary of specific implementation changes to achieve the above:

  * release_on_unlock field, ref_set_release_on_unlock helper, and logic
    to "release on unlock" based on that field are removed

  * The anonymous active_lock struct used by bpf_verifier_state is
    pulled out into a named struct bpf_active_lock.

  * NON_OWN_REF type flag is introduced along with verifier logic
    changes to handle non-owning refs

  * Helpers are added to use NON_OWN_REF flag to implement non-owning
    ref semantics as described above
    * invalidate_non_owning_refs - helper to clobber all non-owning refs
      matching a particular bpf_active_lock identity. Replaces
      release_on_unlock logic in process_spin_lock.
    * ref_set_non_owning - set NON_OWN_REF type flag after doing some
      sanity checking
    * ref_convert_owning_non_owning - convert owning reference w/
      specified ref_obj_id to non-owning references. Set NON_OWN_REF
      flag for each reg with that ref_obj_id and 0-out its ref_obj_id

  * Update linked_list selftests to account for minor semantic
    differences introduced by this patch
    * Writes to a release_on_unlock node ref are not allowed, while
      writes to non-owning reference pointees are. As a result the
      linked_list "write after push" failure tests are no longer scenarios
      that should fail.
    * The test##missing_lock##op and test##incorrect_lock##op
      macro-generated failure tests need to have a valid node argument in
      order to have the same error output as before. Otherwise
      verification will fail early and the expected error output won't be seen.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230212092715.1422619-2-davemarchevsky@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -181,6 +181,7 @@ enum btf_field_type {
 	BPF_KPTR       = BPF_KPTR_UNREF | BPF_KPTR_REF,
 	BPF_LIST_HEAD  = (1 << 4),
 	BPF_LIST_NODE  = (1 << 5),
+	BPF_GRAPH_NODE_OR_ROOT = BPF_LIST_NODE | BPF_LIST_HEAD,
 };
 
 struct btf_field_kptr {
@@ -576,6 +577,11 @@ enum bpf_type_flag {
 	/* MEM is tagged with rcu and memory access needs rcu_read_lock protection. */
 	MEM_RCU			= BIT(13 + BPF_BASE_TYPE_BITS),
 
+	/* Used to tag PTR_TO_BTF_ID | MEM_ALLOC references which are non-owning.
+	 * Currently only valid for linked-list and rbtree nodes.
+	 */
+	NON_OWN_REF		= BIT(14 + BPF_BASE_TYPE_BITS),
+
 	__BPF_TYPE_FLAG_MAX,
 	__BPF_TYPE_LAST_FLAG	= __BPF_TYPE_FLAG_MAX - 1,
 };

include/linux/bpf_verifier.h

@@ -43,6 +43,22 @@ enum bpf_reg_liveness {
 	REG_LIVE_DONE = 0x8, /* liveness won't be updating this register anymore */
 };
 
+/* For every reg representing a map value or allocated object pointer,
+ * we consider the tuple of (ptr, id) for them to be unique in verifier
+ * context and conside them to not alias each other for the purposes of
+ * tracking lock state.
+ */
+struct bpf_active_lock {
+	/* This can either be reg->map_ptr or reg->btf. If ptr is NULL,
+	 * there's no active lock held, and other fields have no
+	 * meaning. If non-NULL, it indicates that a lock is held and
+	 * id member has the reg->id of the register which can be >= 0.
+	 */
+	void *ptr;
+	/* This will be reg->id */
+	u32 id;
+};
+
 struct bpf_reg_state {
 	/* Ordering of fields matters.  See states_equal() */
 	enum bpf_reg_type type;
@@ -226,11 +242,6 @@ struct bpf_reference_state {
 	 * exiting a callback function.
 	 */
 	int callback_ref;
-	/* Mark the reference state to release the registers sharing the same id
-	 * on bpf_spin_unlock (for nodes that we will lose ownership to but are
-	 * safe to access inside the critical section).
-	 */
-	bool release_on_unlock;
 };
 
 /* state of the program:
@@ -331,21 +342,8 @@ struct bpf_verifier_state {
 	u32 branches;
 	u32 insn_idx;
 	u32 curframe;
-	/* For every reg representing a map value or allocated object pointer,
-	 * we consider the tuple of (ptr, id) for them to be unique in verifier
-	 * context and conside them to not alias each other for the purposes of
-	 * tracking lock state.
-	 */
-	struct {
-		/* This can either be reg->map_ptr or reg->btf. If ptr is NULL,
-		 * there's no active lock held, and other fields have no
-		 * meaning. If non-NULL, it indicates that a lock is held and
-		 * id member has the reg->id of the register which can be >= 0.
-		 */
-		void *ptr;
-		/* This will be reg->id */
-		u32 id;
-	} active_lock;
+
+	struct bpf_active_lock active_lock;
 	bool speculative;
 	bool active_rcu_lock;
 

kernel/bpf/verifier.c

@@ -190,6 +190,9 @@ struct bpf_verifier_stack_elem {
 
 static int acquire_reference_state(struct bpf_verifier_env *env, int insn_idx);
 static int release_reference(struct bpf_verifier_env *env, int ref_obj_id);
+static void invalidate_non_owning_refs(struct bpf_verifier_env *env);
+static int ref_set_non_owning(struct bpf_verifier_env *env,
+			      struct bpf_reg_state *reg);
 
 static bool bpf_map_ptr_poisoned(const struct bpf_insn_aux_data *aux)
 {
@@ -457,6 +460,11 @@ static bool type_is_ptr_alloc_obj(u32 type)
 	return base_type(type) == PTR_TO_BTF_ID && type_flag(type) & MEM_ALLOC;
 }
 
+static bool type_is_non_owning_ref(u32 type)
+{
+	return type_is_ptr_alloc_obj(type) && type_flag(type) & NON_OWN_REF;
+}
+
 static struct btf_record *reg_btf_record(const struct bpf_reg_state *reg)
 {
 	struct btf_record *rec = NULL;
@@ -1073,6 +1081,8 @@ static void print_verifier_state(struct bpf_verifier_env *env,
 				verbose_a("id=%d", reg->id);
 			if (reg->ref_obj_id)
 				verbose_a("ref_obj_id=%d", reg->ref_obj_id);
+			if (type_is_non_owning_ref(reg->type))
+				verbose_a("%s", "non_own_ref");
 			if (t != SCALAR_VALUE)
 				verbose_a("off=%d", reg->off);
 			if (type_is_pkt_pointer(t))
@@ -5052,7 +5062,8 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 			return -EACCES;
 		}
 
-		if (type_is_alloc(reg->type) && !reg->ref_obj_id) {
+		if (type_is_alloc(reg->type) && !type_is_non_owning_ref(reg->type) &&
+		    !reg->ref_obj_id) {
 			verbose(env, "verifier internal error: ref_obj_id for allocated object must be non-zero\n");
 			return -EFAULT;
 		}
@@ -6042,9 +6053,7 @@ static int process_spin_lock(struct bpf_verifier_env *env, int regno,
 			cur->active_lock.ptr = btf;
 		cur->active_lock.id = reg->id;
 	} else {
-		struct bpf_func_state *fstate = cur_func(env);
 		void *ptr;
-		int i;
 
 		if (map)
 			ptr = map;
@@ -6060,25 +6069,11 @@ static int process_spin_lock(struct bpf_verifier_env *env, int regno,
 			verbose(env, "bpf_spin_unlock of different lock\n");
 			return -EINVAL;
 		}
-		cur->active_lock.ptr = NULL;
-		cur->active_lock.id = 0;
 
-		for (i = fstate->acquired_refs - 1; i >= 0; i--) {
-			int err;
+		invalidate_non_owning_refs(env);
 
-			/* Complain on error because this reference state cannot
-			 * be freed before this point, as bpf_spin_lock critical
-			 * section does not allow functions that release the
-			 * allocated object immediately.
-			 */
-			if (!fstate->refs[i].release_on_unlock)
-				continue;
-			err = release_reference(env, fstate->refs[i].id);
-			if (err) {
-				verbose(env, "failed to release release_on_unlock reference");
-				return err;
-			}
-		}
+		cur->active_lock.ptr = NULL;
+		cur->active_lock.id = 0;
 	}
 	return 0;
 }
@@ -6546,6 +6541,23 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
 	return 0;
 }
 
+static struct btf_field *
+reg_find_field_offset(const struct bpf_reg_state *reg, s32 off, u32 fields)
+{
+	struct btf_field *field;
+	struct btf_record *rec;
+
+	rec = reg_btf_record(reg);
+	if (!rec)
+		return NULL;
+
+	field = btf_record_find(rec, off, fields);
+	if (!field)
+		return NULL;
+
+	return field;
+}
+
 int check_func_arg_reg_off(struct bpf_verifier_env *env,
 			   const struct bpf_reg_state *reg, int regno,
 			   enum bpf_arg_type arg_type)
@@ -6567,6 +6579,18 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 		 */
 		if (arg_type_is_dynptr(arg_type) && type == PTR_TO_STACK)
 			return 0;
+
+		if ((type_is_ptr_alloc_obj(type) || type_is_non_owning_ref(type)) && reg->off) {
+			if (reg_find_field_offset(reg, reg->off, BPF_GRAPH_NODE_OR_ROOT))
+				return __check_ptr_off_reg(env, reg, regno, true);
+
+			verbose(env, "R%d must have zero offset when passed to release func\n",
+				regno);
+			verbose(env, "No graph node or root found at R%d type:%s off:%d\n", regno,
+				kernel_type_name(reg->btf, reg->btf_id), reg->off);
+			return -EINVAL;
+		}
+
 		/* Doing check_ptr_off_reg check for the offset will catch this
 		 * because fixed_off_ok is false, but checking here allows us
 		 * to give the user a better error message.
@@ -6601,6 +6625,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	case PTR_TO_BTF_ID | PTR_TRUSTED:
 	case PTR_TO_BTF_ID | MEM_RCU:
 	case PTR_TO_BTF_ID | MEM_ALLOC | PTR_TRUSTED:
+	case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF:
 		/* When referenced PTR_TO_BTF_ID is passed to release function,
 		 * its fixed offset must be 0. In the other cases, fixed offset
 		 * can be non-zero. This was already checked above. So pass
@@ -7363,6 +7388,17 @@ static int release_reference(struct bpf_verifier_env *env,
 	return 0;
 }
 
+static void invalidate_non_owning_refs(struct bpf_verifier_env *env)
+{
+	struct bpf_func_state *unused;
+	struct bpf_reg_state *reg;
+
+	bpf_for_each_reg_in_vstate(env->cur_state, unused, reg, ({
+		if (type_is_non_owning_ref(reg->type))
+			__mark_reg_unknown(env, reg);
+	}));
+}
+
 static void clear_caller_saved_regs(struct bpf_verifier_env *env,
 				    struct bpf_reg_state *regs)
 {
@@ -8915,38 +8951,54 @@ static int process_kf_arg_ptr_to_kptr(struct bpf_verifier_env *env,
 	return 0;
 }
 
-static int ref_set_release_on_unlock(struct bpf_verifier_env *env, u32 ref_obj_id)
+static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
 {
-	struct bpf_func_state *state = cur_func(env);
+	struct bpf_verifier_state *state = env->cur_state;
+
+	if (!state->active_lock.ptr) {
+		verbose(env, "verifier internal error: ref_set_non_owning w/o active lock\n");
+		return -EFAULT;
+	}
+
+	if (type_flag(reg->type) & NON_OWN_REF) {
+		verbose(env, "verifier internal error: NON_OWN_REF already set\n");
+		return -EFAULT;
+	}
+
+	reg->type |= NON_OWN_REF;
+	return 0;
+}
+
+static int ref_convert_owning_non_owning(struct bpf_verifier_env *env, u32 ref_obj_id)
+{
+	struct bpf_func_state *state, *unused;
 	struct bpf_reg_state *reg;
 	int i;
 
-	/* bpf_spin_lock only allows calling list_push and list_pop, no BPF
-	 * subprogs, no global functions. This means that the references would
-	 * not be released inside the critical section but they may be added to
-	 * the reference state, and the acquired_refs are never copied out for a
-	 * different frame as BPF to BPF calls don't work in bpf_spin_lock
-	 * critical sections.
-	 */
+	state = cur_func(env);
+
 	if (!ref_obj_id) {
-		verbose(env, "verifier internal error: ref_obj_id is zero for release_on_unlock\n");
+		verbose(env, "verifier internal error: ref_obj_id is zero for "
+			     "owning -> non-owning conversion\n");
 		return -EFAULT;
 	}
+
 	for (i = 0; i < state->acquired_refs; i++) {
-		if (state->refs[i].id == ref_obj_id) {
-			if (state->refs[i].release_on_unlock) {
-				verbose(env, "verifier internal error: expected false release_on_unlock");
-				return -EFAULT;
+		if (state->refs[i].id != ref_obj_id)
+			continue;
+
+		/* Clear ref_obj_id here so release_reference doesn't clobber
+		 * the whole reg
+		 */
+		bpf_for_each_reg_in_vstate(env->cur_state, unused, reg, ({
+			if (reg->ref_obj_id == ref_obj_id) {
+				reg->ref_obj_id = 0;
+				ref_set_non_owning(env, reg);
 			}
-			state->refs[i].release_on_unlock = true;
-			/* Now mark everyone sharing same ref_obj_id as untrusted */
-			bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
-				if (reg->ref_obj_id == ref_obj_id)
-					reg->type |= PTR_UNTRUSTED;
-			}));
-			return 0;
-		}
+		}));
+		return 0;
 	}
+
 	verbose(env, "verifier internal error: ref state missing for ref_obj_id\n");
 	return -EFAULT;
 }
@@ -9081,7 +9133,6 @@ static int process_kf_arg_ptr_to_list_node(struct bpf_verifier_env *env,
 {
 	const struct btf_type *et, *t;
 	struct btf_field *field;
-	struct btf_record *rec;
 	u32 list_node_off;
 
 	if (meta->btf != btf_vmlinux ||
@@ -9098,9 +9149,8 @@ static int process_kf_arg_ptr_to_list_node(struct bpf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	rec = reg_btf_record(reg);
 	list_node_off = reg->off + reg->var_off.value;
-	field = btf_record_find(rec, list_node_off, BPF_LIST_NODE);
+	field = reg_find_field_offset(reg, list_node_off, BPF_LIST_NODE);
 	if (!field || field->offset != list_node_off) {
 		verbose(env, "bpf_list_node not found at offset=%u\n", list_node_off);
 		return -EINVAL;
@@ -9126,8 +9176,8 @@ static int process_kf_arg_ptr_to_list_node(struct bpf_verifier_env *env,
 			btf_name_by_offset(field->graph_root.btf, et->name_off));
 		return -EINVAL;
 	}
-	/* Set arg#1 for expiration after unlock */
-	return ref_set_release_on_unlock(env, reg->ref_obj_id);
+
+	return 0;
 }
 
 static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_arg_meta *meta)
@@ -9406,11 +9456,11 @@ 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;
+	u32 i, nargs, func_id, ptr_type_id, release_ref_obj_id;
 	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;
 	const struct btf_param *args;
 	const struct btf_type *ret_t;
@@ -9505,6 +9555,24 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		}
 	}
 
+	if (meta.func_id == special_kfunc_list[KF_bpf_list_push_front] ||
+	    meta.func_id == special_kfunc_list[KF_bpf_list_push_back]) {
+		release_ref_obj_id = regs[BPF_REG_2].ref_obj_id;
+		err = ref_convert_owning_non_owning(env, release_ref_obj_id);
+		if (err) {
+			verbose(env, "kfunc %s#%d conversion of owning ref to non-owning failed\n",
+				func_name, func_id);
+			return err;
+		}
+
+		err = release_reference(env, release_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]);
 
@@ -11825,8 +11893,10 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 		 */
 		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value || !tnum_equals_const(reg->var_off, 0)))
 			return;
-		if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC | PTR_MAYBE_NULL) && WARN_ON_ONCE(reg->off))
+		if (!(type_is_ptr_alloc_obj(reg->type) || type_is_non_owning_ref(reg->type)) &&
+		    WARN_ON_ONCE(reg->off))
 			return;
+
 		if (is_null) {
 			reg->type = SCALAR_VALUE;
 			/* We don't need id and ref_obj_id from this point

tools/testing/selftests/bpf/prog_tests/linked_list.c

@@ -78,8 +78,6 @@ static struct {
 	{ "direct_write_head", "direct access to bpf_list_head is disallowed" },
 	{ "direct_read_node", "direct access to bpf_list_node is disallowed" },
 	{ "direct_write_node", "direct access to bpf_list_node is disallowed" },
-	{ "write_after_push_front", "only read is supported" },
-	{ "write_after_push_back", "only read is supported" },
 	{ "use_after_unlock_push_front", "invalid mem access 'scalar'" },
 	{ "use_after_unlock_push_back", "invalid mem access 'scalar'" },
 	{ "double_push_front", "arg#1 expected pointer to allocated object" },