Re: [PATCH bpf-next v5 10/16] bpf: Reject stack arguments if tail call reachable

On 4/16/26 9:08 PM, [email protected] wrote:
> Sashiko AI review found 2 potential issue(s):
> - [Critical] False negative allowing tail calls and stack arguments together, leading to JIT memory corruption
> - [High] State leak in verifier DFS causing false positive rejections of valid BPF programs
> --
>
> commit df24720ae892549300f1910df635621a3875dbdf
> Author: Yonghong Song <[email protected]>
>
> bpf: Reject stack arguments if tail call reachable
>
> This commit updates the verifier to reject BPF programs if they contain both
> tail calls and stack arguments. This is enforced during the maximum stack
> depth check traversal.
>
>> Tailcalls are been deprecated. So reject stack arguments
>> if tail call is in the way.
> This isn't a bug, but there is a grammatical error here. "are been" should
> likely be "have been".
>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 7a65b532e84a8..739c3127520c3 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -5662,6 +5662,11 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx,
>>   				verbose(env, "cannot tail call within exception cb\n");
>>   				return -EINVAL;
>>   			}
>> +			if (subprog[tmp].incoming_stack_arg_depth ||
>> +			    subprog[tmp].outgoing_stack_arg_depth) {
>> +				verbose(env, "tail_calls are not allowed in programs with stack args\n");
>> +				return -EINVAL;
>> +			}
> Looking at the earlier parts of check_max_stack_depth_subprog(), the local
> variable tail_call_reachable only appears to be set to true when jumping
> into a callee that has a tail call:
>
>      if (subprog[idx].has_tail_call)
>          tail_call_reachable = true;
>
> If the main program (subprog[0]) natively contains a tail call,
> tail_call_reachable would remain false, causing this entire check block to
> be skipped.
>
> Does this logic successfully catch the case where the main program itself
> contains a tail call and also uses stack arguments?

Yes, we do have an issue here. Will fix.

>
> If this escapes the verifier, the JIT might emit prologue tail call
> instructions (pushing 24 bytes on x86) while calculating stack argument
> offsets assuming no tail call pushes, overwriting the BPF program stack.
>
> Could this regression lead to memory corruption in the JIT?
>
> When the traversal pops a frame to return to the caller and jumps back to
> continue_func, the tail_call_reachable variable is not reset to false:
>
>      /* pop caller idx from callee */
>      idx = dinfo[idx].caller;
>      ...
>      goto continue_func;
>
> If the verifier traverses a subprogram with a tail call, returns, and then
> traverses a completely unrelated sibling subprogram that uses stack
> arguments, it seems it would incorrectly evaluate tail_call_reachable as
> true and reject the valid sibling.
>
> Is there a state leak in this DFS traversal that could cause false positive
> rejections of valid BPF programs?
>
>>   			subprog[tmp].tail_call_reachable = true;
>>   		}
>>   	if (subprog[0].tail_call_reachable)