[PATCH bpf-next v2] bpf: Remove redundant free_verifier_state()/pop_stack()

[Luis Gerhorst]

This patch removes duplicated code.

Eduard points out [1]:

    Same cleanup cycles are done in push_stack() and push_async_cb(),
    both functions are only reachable from do_check_common() via
    do_check() -> do_check_insn().

    Hence, I think that cur state should not be freed in push_*()
    functions and pop_stack() loop there is not needed.

This would also fix the 'symptom' for [2], but the issue also has a
simpler fix which was sent separately. This fix also makes sure the
push_*() callers always return an error for which
error_recoverable_with_nospec(err) is false. This is required because
otherwise we try to recover and access the stale `state`.

Moving free_verifier_state() and pop_stack(..., pop_log=false) to happen
after the bpf_vlog_reset() call in do_check_common() is fine because the
pop_stack() call that is moved does not call bpf_vlog_reset() with the
pop_log=false parameter.

[1] https://lore.kernel.org/all/b6931bd0dd72327c55287862f821ca6c4c3eb69a.camel@xxxxxxxxx/
[2] https://lore.kernel.org/all/68497853.050a0220.33aa0e.036a.GAE@xxxxxxxxxx/

Reported-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Link: https://lore.kernel.org/all/b6931bd0dd72327c55287862f821ca6c4c3eb69a.camel@xxxxxxxxx/
Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Signed-off-by: Luis Gerhorst <luis.gerhorst@xxxxxx>
---

Changes since v1:
- Move free_verifier_state() and pop_stack() into free_state() and
  remove err label in push_*() altogether (incl. comment), both
  suggested by Eduard
- Link to v1: https://lore.kernel.org/bpf/20250611211431.275731-1-luis.gerhorst@xxxxxx/

 kernel/bpf/verifier.c | 37 +++++++++++--------------------------
 1 file changed, 11 insertions(+), 26 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c378074516cf..5f4e0a8b20f8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2097,7 +2097,7 @@ static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
 
 	elem = kzalloc(sizeof(struct bpf_verifier_stack_elem), GFP_KERNEL);
 	if (!elem)
-		goto err;
+		return NULL;
 
 	elem->insn_idx = insn_idx;
 	elem->prev_insn_idx = prev_insn_idx;
@@ -2107,12 +2107,12 @@ static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
 	env->stack_size++;
 	err = copy_verifier_state(&elem->st, cur);
 	if (err)
-		goto err;
+		return NULL;
 	elem->st.speculative |= speculative;
 	if (env->stack_size > BPF_COMPLEXITY_LIMIT_JMP_SEQ) {
 		verbose(env, "The sequence of %d jumps is too complex.\n",
 			env->stack_size);
-		goto err;
+		return NULL;
 	}
 	if (elem->st.parent) {
 		++elem->st.parent->branches;
@@ -2127,12 +2127,6 @@ static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
 		 */
 	}
 	return &elem->st;
-err:
-	free_verifier_state(env->cur_state, true);
-	env->cur_state = NULL;
-	/* pop all elements and return */
-	while (!pop_stack(env, NULL, NULL, false));
-	return NULL;
 }
 
 #define CALLER_SAVED_REGS 6
@@ -2864,7 +2858,7 @@ static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
 
 	elem = kzalloc(sizeof(struct bpf_verifier_stack_elem), GFP_KERNEL);
 	if (!elem)
-		goto err;
+		return NULL;
 
 	elem->insn_idx = insn_idx;
 	elem->prev_insn_idx = prev_insn_idx;
@@ -2876,7 +2870,7 @@ static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
 		verbose(env,
 			"The sequence of %d jumps is too complex for async cb.\n",
 			env->stack_size);
-		goto err;
+		return NULL;
 	}
 	/* Unlike push_stack() do not copy_verifier_state().
 	 * The caller state doesn't matter.
@@ -2887,19 +2881,13 @@ static struct bpf_verifier_state *push_async_cb(struct bpf_verifier_env *env,
 	elem->st.in_sleepable = is_sleepable;
 	frame = kzalloc(sizeof(*frame), GFP_KERNEL);
 	if (!frame)
-		goto err;
+		return NULL;
 	init_func_state(env, frame,
 			BPF_MAIN_FUNC /* callsite */,
 			0 /* frameno within this callchain */,
 			subprog /* subprog number within this prog */);
 	elem->st.frame[0] = frame;
 	return &elem->st;
-err:
-	free_verifier_state(env->cur_state, true);
-	env->cur_state = NULL;
-	/* pop all elements and return */
-	while (!pop_stack(env, NULL, NULL, false));
-	return NULL;
 }
 
 
@@ -22934,6 +22922,11 @@ static void free_states(struct bpf_verifier_env *env)
 	struct bpf_scc_info *info;
 	int i, j;
 
+	WARN_ON_ONCE(!env->cur_state);
+	free_verifier_state(env->cur_state, true);
+	env->cur_state = NULL;
+	while (!pop_stack(env, NULL, NULL, false));
+
 	list_for_each_safe(pos, tmp, &env->free_list) {
 		sl = container_of(pos, struct bpf_verifier_state_list, node);
 		free_verifier_state(&sl->state, false);
@@ -23085,14 +23078,6 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
 
 	ret = do_check(env);
 out:
-	/* check for NULL is necessary, since cur_state can be freed inside
-	 * do_check() under memory pressure.
-	 */
-	if (env->cur_state) {
-		free_verifier_state(env->cur_state, true);
-		env->cur_state = NULL;
-	}
-	while (!pop_stack(env, NULL, NULL, false));
 	if (!ret && pop_log)
 		bpf_vlog_reset(&env->log, 0);
 	free_states(env);

base-commit: af91af33c16853c569ca814124781b849886f007
-- 
2.49.0