[PATCH bpf-next v7 3/9] bpf: Check potential private stack recursion for progs with async callback

[Yonghong Song <yonghong.song@linux.dev>]

In previous patch, tracing progs are enabled for private stack since
recursion checking ensures there exists no nested same bpf prog run on
the same cpu.

But it is still possible for nested bpf subprog run on the same cpu
if the same subprog is called in both main prog and async callback,
or in different async callbacks. For example,
  main_prog
   bpf_timer_set_callback(timer, timer_cb);
   call sub1
  sub1
   ...
  time_cb
   call sub1

In the above case, nested subprog run for sub1 is possible with one in
process context and the other in softirq context. If this is the case,
the verifier will disable private stack for this bpf prog.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 kernel/bpf/verifier.c | 46 ++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 41 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d3f4cbab97bc..596afd29f088 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -6070,7 +6070,8 @@ static int round_up_stack_depth(struct bpf_verifier_env *env, int stack_depth)
  */
 static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
 					 int *subtree_depth, int *depth_frame,
-					 int priv_stack_supported)
+					 int priv_stack_supported,
+					 char *subprog_visited)
 {
 	struct bpf_subprog_info *subprog = env->subprog_info;
 	struct bpf_insn *insn = env->prog->insnsi;
@@ -6120,8 +6121,12 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
 					idx, subprog_depth);
 				return -EACCES;
 			}
-			if (subprog_depth >= BPF_PRIV_STACK_MIN_SIZE)
+			if (subprog_depth >= BPF_PRIV_STACK_MIN_SIZE) {
 				subprog[idx].use_priv_stack = true;
+				subprog_visited[idx] = 1;
+			}
+		} else {
+			subprog_visited[idx] = 1;
 		}
 	}
 continue_func:
@@ -6222,19 +6227,42 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
 static int check_max_stack_depth(struct bpf_verifier_env *env)
 {
 	struct bpf_subprog_info *si = env->subprog_info;
+	char *subprogs1 = NULL, *subprogs2 = NULL;
 	int ret, subtree_depth = 0, depth_frame;
+	int orig_priv_stack_supported;
 	int priv_stack_supported;
 
 	priv_stack_supported = bpf_enable_priv_stack(env);
 	if (priv_stack_supported < 0)
 		return priv_stack_supported;
 
+	orig_priv_stack_supported = priv_stack_supported;
+	if (orig_priv_stack_supported != NO_PRIV_STACK) {
+		subprogs1 = kvmalloc(env->subprog_cnt * 2, __GFP_ZERO);
+		if (!subprogs1)
+			priv_stack_supported = NO_PRIV_STACK;
+		else
+			subprogs2 = subprogs1 + env->subprog_cnt;
+	}
+
 	for (int i = 0; i < env->subprog_cnt; i++) {
 		if (!i || si[i].is_async_cb) {
 			ret = check_max_stack_depth_subprog(env, i, &subtree_depth, &depth_frame,
-							    priv_stack_supported);
+							    priv_stack_supported, subprogs2);
 			if (ret < 0)
-				return ret;
+				goto out;
+
+			if (priv_stack_supported != NO_PRIV_STACK) {
+				for (int j = 0; j < env->subprog_cnt; j++) {
+					if (subprogs1[j] && subprogs2[j]) {
+						priv_stack_supported = NO_PRIV_STACK;
+						break;
+					}
+					subprogs1[j] |= subprogs2[j];
+				}
+			}
+			if (priv_stack_supported != NO_PRIV_STACK)
+				memset(subprogs2, 0, env->subprog_cnt);
 		}
 	}
 	if (priv_stack_supported == NO_PRIV_STACK) {
@@ -6243,10 +6271,18 @@ static int check_max_stack_depth(struct bpf_verifier_env *env)
 				depth_frame, subtree_depth);
 			return -EACCES;
 		}
+		if (orig_priv_stack_supported == PRIV_STACK_ADAPTIVE) {
+			for (int i = 0; i < env->subprog_cnt; i++)
+				si[i].use_priv_stack = false;
+		}
 	}
 	if (si[0].use_priv_stack)
 		env->prog->aux->use_priv_stack = true;
-	return 0;
+	ret = 0;
+
+out:
+	kvfree(subprogs1);
+	return ret;
 }
 
 #ifndef CONFIG_BPF_JIT_ALWAYS_ON
-- 
2.43.5