[bpf,v2,2/3] bpf: Repeat check_max_stack_depth for async callbacks

Commit Message

Kumar Kartikeya Dwivedi July 17, 2023, 4:15 p.m. UTC

While the check_max_stack_depth function explores call chains emanating
from the main prog, which is typically enough to cover all possible call
chains, it doesn't explore those rooted at async callbacks unless the
async callback will have been directly called, since unlike non-async
callbacks it skips their instruction exploration as they don't
contribute to stack depth.

It could be the case that the async callback leads to a callchain which
exceeds the stack depth, but this is never reachable while only
exploring the entry point from main subprog. Hence, repeat the check for
the main subprog *and* all async callbacks marked by the symbolic
execution pass of the verifier, as execution of the program may begin at
any of them.

Consider functions with following stack depths:
main: 256
async: 256
foo: 256

main:
    rX = async
    bpf_timer_set_callback(...)

async:
    foo()

Here, async is not descended as it does not contribute to stack depth of
main (since it is referenced using bpf_pseudo_func and not
bpf_pseudo_call). However, when async is invoked asynchronously, it will
end up breaching the MAX_BPF_STACK limit by calling foo.

Hence, in addition to main, we also need to explore call chains
beginning at all async callback subprogs in a program.

Fixes: 7ddc80a476c2 ("bpf: Teach stack depth check about async callbacks.")
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
---
 kernel/bpf/verifier.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

Comments

Alexei Starovoitov July 18, 2023, 10:29 p.m. UTC | #1

On Mon, Jul 17, 2023 at 9:15 AM Kumar Kartikeya Dwivedi
<[e-mail]> wrote:
>
> While the check_max_stack_depth function explores call chains emanating
> from the main prog, which is typically enough to cover all possible call
> chains, it doesn't explore those rooted at async callbacks unless the
> async callback will have been directly called, since unlike non-async
> callbacks it skips their instruction exploration as they don't
> contribute to stack depth.
>
> It could be the case that the async callback leads to a callchain which
> exceeds the stack depth, but this is never reachable while only
> exploring the entry point from main subprog. Hence, repeat the check for
> the main subprog *and* all async callbacks marked by the symbolic
> execution pass of the verifier, as execution of the program may begin at
> any of them.
>
> Consider functions with following stack depths:
> main: 256
> async: 256
> foo: 256
>
> main:
>     rX = async
>     bpf_timer_set_callback(...)
>
> async:
>     foo()
>
> Here, async is not descended as it does not contribute to stack depth of
> main (since it is referenced using bpf_pseudo_func and not
> bpf_pseudo_call). However, when async is invoked asynchronously, it will
> end up breaching the MAX_BPF_STACK limit by calling foo.
>
> Hence, in addition to main, we also need to explore call chains
> beginning at all async callback subprogs in a program.
>
> Fixes: 7ddc80a476c2 ("bpf: Teach stack depth check about async callbacks.")
> Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>

Applied. Thanks

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e682056dd144..02a021c524ab 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5573,16 +5573,17 @@  static int update_stack_depth(struct bpf_verifier_env *env,
  * 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)
+static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx)
 {
-	int depth = 0, frame = 0, idx = 0, i = 0, subprog_end;
 	struct bpf_subprog_info *subprog = env->subprog_info;
 	struct bpf_insn *insn = env->prog->insnsi;
+	int depth = 0, frame = 0, i, subprog_end;
 	bool tail_call_reachable = false;
 	int ret_insn[MAX_CALL_FRAMES];
 	int ret_prog[MAX_CALL_FRAMES];
 	int j;
 
+	i = subprog[idx].start;
 process_func:
 	/* protect against potential stack overflow that might happen when
 	 * bpf2bpf calls get combined with tailcalls. Limit the caller's stack
@@ -5683,6 +5684,22 @@  static int check_max_stack_depth(struct bpf_verifier_env *env)
 	goto continue_func;
 }
 
+static int check_max_stack_depth(struct bpf_verifier_env *env)
+{
+	struct bpf_subprog_info *si = env->subprog_info;
+	int ret;
+
+	for (int i = 0; i < env->subprog_cnt; i++) {
+		if (!i || si[i].is_async_cb) {
+			ret = check_max_stack_depth_subprog(env, i);
+			if (ret < 0)
+				return ret;
+		}
+		continue;
+	}
+	return 0;
+}
+
 #ifndef CONFIG_BPF_JIT_ALWAYS_ON
 static int get_callee_stack_depth(struct bpf_verifier_env *env,
 				  const struct bpf_insn *insn, int idx)