bpf: fix maximum stack depth tracking logic

Instead of computing max stack depth for current call chain
during the main verifier pass track stack depth of each
function independently and after do_check() is done do
another pass over all instructions analyzing depth
of all possible call stacks.

Fixes: f4d7e40a ("bpf: introduce function calls (verification)")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -194,6 +194,7 @@ struct bpf_verifier_env {
 	struct bpf_insn_aux_data *insn_aux_data; /* array of per-insn state */
 	struct bpf_verifer_log log;
 	u32 subprog_starts[BPF_MAX_SUBPROGS];
+	/* computes the stack depth of each bpf function */
 	u16 subprog_stack_depth[BPF_MAX_SUBPROGS + 1];
 	u32 subprog_cnt;
 };

kernel/bpf/verifier.c

@@ -1430,33 +1430,80 @@ static int update_stack_depth(struct bpf_verifier_env *env,
 			      const struct bpf_func_state *func,
 			      int off)
 {
-	u16 stack = env->subprog_stack_depth[func->subprogno], total = 0;
-	struct bpf_verifier_state *cur = env->cur_state;
-	int i;
+	u16 stack = env->subprog_stack_depth[func->subprogno];
 
 	if (stack >= -off)
 		return 0;
 
 	/* update known max for given subprogram */
 	env->subprog_stack_depth[func->subprogno] = -off;
+	return 0;
+}
 
-	/* compute the total for current call chain */
-	for (i = 0; i <= cur->curframe; i++) {
-		u32 depth = env->subprog_stack_depth[cur->frame[i]->subprogno];
-
-		/* round up to 32-bytes, since this is granularity
-		 * of interpreter stack sizes
-		 */
-		depth = round_up(depth, 32);
-		total += depth;
-	}
+/* starting from main bpf function walk all instructions of the function
+ * and recursively walk all callees that given function can call.
+ * Ignore jump and exit insns.
+ * Since recursion is prevented by check_cfg() this algorithm
+ * only needs a local stack of MAX_CALL_FRAMES to remember callsites
+ */
+static int check_max_stack_depth(struct bpf_verifier_env *env)
+{
+	int depth = 0, frame = 0, subprog = 0, i = 0, subprog_end;
+	struct bpf_insn *insn = env->prog->insnsi;
+	int insn_cnt = env->prog->len;
+	int ret_insn[MAX_CALL_FRAMES];
+	int ret_prog[MAX_CALL_FRAMES];
 
-	if (total > MAX_BPF_STACK) {
+process_func:
+	/* round up to 32-bytes, since this is granularity
+	 * of interpreter stack size
+	 */
+	depth += round_up(max_t(u32, env->subprog_stack_depth[subprog], 1), 32);
+	if (depth > MAX_BPF_STACK) {
 		verbose(env, "combined stack size of %d calls is %d. Too large\n",
-			cur->curframe, total);
+			frame + 1, depth);
 		return -EACCES;
 	}
-	return 0;
+continue_func:
+	if (env->subprog_cnt == subprog)
+		subprog_end = insn_cnt;
+	else
+		subprog_end = env->subprog_starts[subprog];
+	for (; i < subprog_end; i++) {
+		if (insn[i].code != (BPF_JMP | BPF_CALL))
+			continue;
+		if (insn[i].src_reg != BPF_PSEUDO_CALL)
+			continue;
+		/* remember insn and function to return to */
+		ret_insn[frame] = i + 1;
+		ret_prog[frame] = subprog;
+
+		/* find the callee */
+		i = i + insn[i].imm + 1;
+		subprog = find_subprog(env, i);
+		if (subprog < 0) {
+			WARN_ONCE(1, "verifier bug. No program starts at insn %d\n",
+				  i);
+			return -EFAULT;
+		}
+		subprog++;
+		frame++;
+		if (frame >= MAX_CALL_FRAMES) {
+			WARN_ONCE(1, "verifier bug. Call stack is too deep\n");
+			return -EFAULT;
+		}
+		goto process_func;
+	}
+	/* end of for() loop means the last insn of the 'subprog'
+	 * was reached. Doesn't matter whether it was JA or EXIT
+	 */
+	if (frame == 0)
+		return 0;
+	depth -= round_up(max_t(u32, env->subprog_stack_depth[subprog], 1), 32);
+	frame--;
+	i = ret_insn[frame];
+	subprog = ret_prog[frame];
+	goto continue_func;
 }
 
 static int get_callee_stack_depth(struct bpf_verifier_env *env,
@@ -5403,6 +5450,9 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr)
 	if (ret == 0)
 		sanitize_dead_code(env);
 
+	if (ret == 0)
+		ret = check_max_stack_depth(env);
+
 	if (ret == 0)
 		/* program is valid, convert *(u32*)(ctx + off) accesses */
 		ret = convert_ctx_accesses(env);