[bpf-next,v2] bpf: Refactor active lock management

Commit Message

Kumar Kartikeya Dwivedi Nov. 3, 2024, 9:22 p.m. UTC

When bpf_spin_lock was introduced originally, there was deliberation on
whether to use an array of lock IDs, but since bpf_spin_lock is limited
to holding a single lock at any given time, we've been using a single ID
to identify the held lock.

In preparation for introducing spin locks that can be taken multiple
times, introduce support for acquiring multiple lock IDs. For this
purpose, reuse the acquired_refs array and store both lock and pointer
references. We tag the entry with REF_TYPE_PTR or REF_TYPE_BPF_LOCK to
disambiguate and find the relevant entry. The ptr field is used to track
the map_ptr or btf (for bpf_obj_new allocations) to ensure locks can be
matched with protected fields within the same "allocation", i.e.
bpf_obj_new object or map value.

The struct active_lock is changed to a boolean as the state is part of
the acquired_refs array, and we only need active_lock as a cheap way
of detecting lock presence.

Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
---
Changelog:
v1 -> v2
v1: https://lore.kernel.org/bpf/[e-mail]

 * Fix refsafe state comparison to check callback_ref and ptr separately.
---
 include/linux/bpf_verifier.h |  34 +++++---
 kernel/bpf/verifier.c        | 148 ++++++++++++++++++++++++++---------
 2 files changed, 131 insertions(+), 51 deletions(-)

--
2.43.5

Patch

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 4513372c5bc8..1e7e1803d78b 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -266,6 +266,10 @@  struct bpf_stack_state {
 };

 struct bpf_reference_state {
+	/* Each reference object has a type. Ensure REF_TYPE_PTR is zero to
+	 * default to pointer reference on zero initialization of a state.
+	 */
+	enum { REF_TYPE_PTR = 0, REF_TYPE_BPF_LOCK } type;
 	/* Track each reference created with a unique id, even if the same
 	 * instruction creates the reference multiple times (eg, via CALL).
 	 */
@@ -274,17 +278,23 @@  struct bpf_reference_state {
 	 * is used purely to inform the user of a reference leak.
 	 */
 	int insn_idx;
-	/* There can be a case like:
-	 * main (frame 0)
-	 *  cb (frame 1)
-	 *   func (frame 3)
-	 *    cb (frame 4)
-	 * Hence for frame 4, if callback_ref just stored boolean, it would be
-	 * impossible to distinguish nested callback refs. Hence store the
-	 * frameno and compare that to callback_ref in check_reference_leak when
-	 * exiting a callback function.
-	 */
-	int callback_ref;
+	union {
+		/* There can be a case like:
+		 * main (frame 0)
+		 *  cb (frame 1)
+		 *   func (frame 3)
+		 *    cb (frame 4)
+		 * Hence for frame 4, if callback_ref just stored boolean, it would be
+		 * impossible to distinguish nested callback refs. Hence store the
+		 * frameno and compare that to callback_ref in check_reference_leak when
+		 * exiting a callback function.
+		 */
+		int callback_ref;
+		/* Use to keep track of the source object of a lock, to ensure
+		 * it matches on unlock.
+		 */
+		void *ptr;
+	};
 };

 struct bpf_retval_range {
@@ -434,7 +444,7 @@  struct bpf_verifier_state {
 	u32 insn_idx;
 	u32 curframe;

-	struct bpf_active_lock active_lock;
+	bool active_lock;
 	bool speculative;
 	bool active_rcu_lock;
 	u32 active_preempt_lock;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 797cf3ed32e0..fb4171941971 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1335,6 +1335,7 @@  static int acquire_reference_state(struct bpf_verifier_env *env, int insn_idx)
 	if (err)
 		return err;
 	id = ++env->id_gen;
+	state->refs[new_ofs].type = REF_TYPE_PTR;
 	state->refs[new_ofs].id = id;
 	state->refs[new_ofs].insn_idx = insn_idx;
 	state->refs[new_ofs].callback_ref = state->in_callback_fn ? state->frameno : 0;
@@ -1342,6 +1343,23 @@  static int acquire_reference_state(struct bpf_verifier_env *env, int insn_idx)
 	return id;
 }

+static int acquire_lock_state(struct bpf_verifier_env *env, int insn_idx, int type, int id, void *ptr)
+{
+	struct bpf_func_state *state = cur_func(env);
+	int new_ofs = state->acquired_refs;
+	int err;
+
+	err = resize_reference_state(state, state->acquired_refs + 1);
+	if (err)
+		return err;
+	state->refs[new_ofs].type = type;
+	state->refs[new_ofs].id = id;
+	state->refs[new_ofs].insn_idx = insn_idx;
+	state->refs[new_ofs].ptr = ptr;
+
+	return 0;
+}
+
 /* release function corresponding to acquire_reference_state(). Idempotent. */
 static int release_reference_state(struct bpf_func_state *state, int ptr_id)
 {
@@ -1349,6 +1367,8 @@  static int release_reference_state(struct bpf_func_state *state, int ptr_id)

 	last_idx = state->acquired_refs - 1;
 	for (i = 0; i < state->acquired_refs; i++) {
+		if (state->refs[i].type != REF_TYPE_PTR)
+			continue;
 		if (state->refs[i].id == ptr_id) {
 			/* Cannot release caller references in callbacks */
 			if (state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
@@ -1364,6 +1384,43 @@  static int release_reference_state(struct bpf_func_state *state, int ptr_id)
 	return -EINVAL;
 }

+static int release_lock_state(struct bpf_func_state *state, int type, int id, void *ptr)
+{
+	int i, last_idx;
+
+	last_idx = state->acquired_refs - 1;
+	for (i = 0; i < state->acquired_refs; i++) {
+		if (state->refs[i].type != type)
+			continue;
+		if (state->refs[i].id == id && state->refs[i].ptr == ptr) {
+			if (last_idx && i != last_idx)
+				memcpy(&state->refs[i], &state->refs[last_idx],
+				       sizeof(*state->refs));
+			memset(&state->refs[last_idx], 0, sizeof(*state->refs));
+			state->acquired_refs--;
+			return 0;
+		}
+	}
+	return -EINVAL;
+}
+
+static struct bpf_reference_state *find_lock_state(struct bpf_verifier_env *env, int id, void *ptr)
+{
+	struct bpf_func_state *state = cur_func(env);
+	int i;
+
+	for (i = 0; i < state->acquired_refs; i++) {
+		struct bpf_reference_state *s = &state->refs[i];
+
+		if (s->type == REF_TYPE_PTR)
+			continue;
+
+		if (s->id == id && s->ptr == ptr)
+			return s;
+	}
+	return NULL;
+}
+
 static void free_func_state(struct bpf_func_state *state)
 {
 	if (!state)
@@ -1430,12 +1487,11 @@  static int copy_verifier_state(struct bpf_verifier_state *dst_state,
 		dst_state->frame[i] = NULL;
 	}
 	dst_state->speculative = src->speculative;
+	dst_state->active_lock = src->active_lock;
 	dst_state->active_rcu_lock = src->active_rcu_lock;
 	dst_state->active_preempt_lock = src->active_preempt_lock;
 	dst_state->in_sleepable = src->in_sleepable;
 	dst_state->curframe = src->curframe;
-	dst_state->active_lock.ptr = src->active_lock.ptr;
-	dst_state->active_lock.id = src->active_lock.id;
 	dst_state->branches = src->branches;
 	dst_state->parent = src->parent;
 	dst_state->first_insn_idx = src->first_insn_idx;
@@ -5423,7 +5479,7 @@  static bool in_sleepable(struct bpf_verifier_env *env)
 static bool in_rcu_cs(struct bpf_verifier_env *env)
 {
 	return env->cur_state->active_rcu_lock ||
-	       env->cur_state->active_lock.ptr ||
+	       env->cur_state->active_lock ||
 	       !in_sleepable(env);
 }

@@ -7698,6 +7754,7 @@  static int process_spin_lock(struct bpf_verifier_env *env, int regno,
 	struct bpf_map *map = NULL;
 	struct btf *btf = NULL;
 	struct btf_record *rec;
+	int err;

 	if (!is_const) {
 		verbose(env,
@@ -7729,16 +7786,27 @@  static int process_spin_lock(struct bpf_verifier_env *env, int regno,
 		return -EINVAL;
 	}
 	if (is_lock) {
-		if (cur->active_lock.ptr) {
+		void *ptr;
+
+		if (map)
+			ptr = map;
+		else
+			ptr = btf;
+
+		if (cur->active_lock) {
 			verbose(env,
 				"Locking two bpf_spin_locks are not allowed\n");
 			return -EINVAL;
 		}
-		if (map)
-			cur->active_lock.ptr = map;
-		else
-			cur->active_lock.ptr = btf;
-		cur->active_lock.id = reg->id;
+		err = acquire_lock_state(env, env->insn_idx, REF_TYPE_BPF_LOCK, reg->id, ptr);
+		if (err < 0) {
+			verbose(env, "Failed to acquire lock state\n");
+			return err;
+		}
+		/* It is not safe to allow multiple bpf_spin_lock calls, so
+		 * disallow them until this lock has been unlocked.
+		 */
+		cur->active_lock = true;
 	} else {
 		void *ptr;

@@ -7747,20 +7815,18 @@  static int process_spin_lock(struct bpf_verifier_env *env, int regno,
 		else
 			ptr = btf;

-		if (!cur->active_lock.ptr) {
+		if (!cur->active_lock) {
 			verbose(env, "bpf_spin_unlock without taking a lock\n");
 			return -EINVAL;
 		}
-		if (cur->active_lock.ptr != ptr ||
-		    cur->active_lock.id != reg->id) {
+
+		if (release_lock_state(cur_func(env), REF_TYPE_BPF_LOCK, reg->id, ptr)) {
 			verbose(env, "bpf_spin_unlock of different lock\n");
 			return -EINVAL;
 		}

 		invalidate_non_owning_refs(env);
-
-		cur->active_lock.ptr = NULL;
-		cur->active_lock.id = 0;
+		cur->active_lock = false;
 	}
 	return 0;
 }
@@ -9818,7 +9884,7 @@  static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		const char *sub_name = subprog_name(env, subprog);

 		/* Only global subprogs cannot be called with a lock held. */
-		if (env->cur_state->active_lock.ptr) {
+		if (env->cur_state->active_lock) {
 			verbose(env, "global function calls are not allowed while holding a lock,\n"
 				     "use static function instead\n");
 			return -EINVAL;
@@ -10343,6 +10409,8 @@  static int check_reference_leak(struct bpf_verifier_env *env, bool exception_exi
 		return 0;

 	for (i = 0; i < state->acquired_refs; i++) {
+		if (state->refs[i].type != REF_TYPE_PTR)
+			continue;
 		if (!exception_exit && state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
 			continue;
 		verbose(env, "Unreleased reference id=%d alloc_insn=%d\n",
@@ -11554,7 +11622,7 @@  static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state
 	struct bpf_verifier_state *state = env->cur_state;
 	struct btf_record *rec = reg_btf_record(reg);

-	if (!state->active_lock.ptr) {
+	if (!state->active_lock) {
 		verbose(env, "verifier internal error: ref_set_non_owning w/o active lock\n");
 		return -EFAULT;
 	}
@@ -11651,6 +11719,7 @@  static int ref_convert_owning_non_owning(struct bpf_verifier_env *env, u32 ref_o
  */
 static int check_reg_allocation_locked(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
 {
+	struct bpf_reference_state *s;
 	void *ptr;
 	u32 id;

@@ -11667,10 +11736,10 @@  static int check_reg_allocation_locked(struct bpf_verifier_env *env, struct bpf_
 	}
 	id = reg->id;

-	if (!env->cur_state->active_lock.ptr)
+	if (!env->cur_state->active_lock)
 		return -EINVAL;
-	if (env->cur_state->active_lock.ptr != ptr ||
-	    env->cur_state->active_lock.id != id) {
+	s = find_lock_state(env, id, ptr);
+	if (!s) {
 		verbose(env, "held lock and object are not in the same allocation\n");
 		return -EINVAL;
 	}
@@ -15786,17 +15855,17 @@  static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
 	 * gen_ld_abs() may terminate the program at runtime, leading to
 	 * reference leak.
 	 */
+	if (env->cur_state->active_lock) {
+		verbose(env, "BPF_LD_[ABS|IND] cannot be used inside bpf_spin_lock-ed region\n");
+		return -EINVAL;
+	}
+
 	err = check_reference_leak(env, false);
 	if (err) {
 		verbose(env, "BPF_LD_[ABS|IND] cannot be mixed with socket references\n");
 		return err;
 	}

-	if (env->cur_state->active_lock.ptr) {
-		verbose(env, "BPF_LD_[ABS|IND] cannot be used inside bpf_spin_lock-ed region\n");
-		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;
@@ -17552,8 +17621,19 @@  static bool refsafe(struct bpf_func_state *old, struct bpf_func_state *cur,
 		return false;

 	for (i = 0; i < old->acquired_refs; i++) {
-		if (!check_ids(old->refs[i].id, cur->refs[i].id, idmap))
+		if (!check_ids(old->refs[i].id, cur->refs[i].id, idmap) ||
+		    old->refs[i].type != cur->refs[i].type)
 			return false;
+		switch (old->refs[i].type) {
+		case REF_TYPE_PTR:
+			if (old->refs[i].callback_ref != cur->refs[i].callback_ref)
+				return false;
+			break;
+		default:
+			if (old->refs[i].ptr != cur->refs[i].ptr)
+				return false;
+			break;
+		}
 	}

 	return true;
@@ -17631,17 +17711,7 @@  static bool states_equal(struct bpf_verifier_env *env,
 	if (old->speculative && !cur->speculative)
 		return false;

-	if (old->active_lock.ptr != cur->active_lock.ptr)
-		return false;
-
-	/* Old and cur active_lock's have to be either both present
-	 * or both absent.
-	 */
-	if (!!old->active_lock.id != !!cur->active_lock.id)
-		return false;
-
-	if (old->active_lock.id &&
-	    !check_ids(old->active_lock.id, cur->active_lock.id, &env->idmap_scratch))
+	if (old->active_lock != cur->active_lock)
 		return false;

 	if (old->active_rcu_lock != cur->active_rcu_lock)
@@ -18542,7 +18612,7 @@  static int do_check(struct bpf_verifier_env *env)
 					return -EINVAL;
 				}

-				if (env->cur_state->active_lock.ptr) {
+				if (env->cur_state->active_lock) {
 					if ((insn->src_reg == BPF_REG_0 && insn->imm != BPF_FUNC_spin_unlock) ||
 					    (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
 					     (insn->off != 0 || !is_bpf_graph_api_kfunc(insn->imm)))) {
@@ -18591,7 +18661,7 @@  static int do_check(struct bpf_verifier_env *env)
 					return -EINVAL;
 				}
 process_bpf_exit_full:
-				if (env->cur_state->active_lock.ptr && !env->cur_state->curframe) {
+				if (env->cur_state->active_lock && !env->cur_state->curframe) {
 					verbose(env, "bpf_spin_unlock is missing\n");
 					return -EINVAL;
 				}