From: Kumar Kartikeya Dwivedi memxor@gmail.com
[ Upstream commit 5415ccd50a8620c8cbaa32d6f18c946c453566f5 ]
The check_max_stack_depth pass happens after the verifier's symbolic execution, and attempts to walk the call graph of the BPF program, ensuring that the stack usage stays within bounds for all possible call chains. There are two cases to consider: bpf_pseudo_func and bpf_pseudo_call. In the former case, the callback pointer is loaded into a register, and is assumed that it is passed to some helper later which calls it (however there is no way to be sure), but the check remains conservative and accounts the stack usage anyway. For this particular case, asynchronous callbacks are skipped as they execute asynchronously when their corresponding event fires.
The case of bpf_pseudo_call is simpler and we know that the call is definitely made, hence the stack depth of the subprog is accounted for.
However, the current check still skips an asynchronous callback even if a bpf_pseudo_call was made for it. This is erroneous, as it will miss accounting for the stack usage of the asynchronous callback, which can be used to breach the maximum stack depth limit.
Fix this by only skipping asynchronous callbacks when the instruction is not a pseudo call to the subprog.
Fixes: 7ddc80a476c2 ("bpf: Teach stack depth check about async callbacks.") Signed-off-by: Kumar Kartikeya Dwivedi memxor@gmail.com Link: https://lore.kernel.org/r/20230705144730.235802-2-memxor@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 7a70595c3c15a..bd31aa6407a78 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -3765,8 +3765,9 @@ static int check_max_stack_depth(struct bpf_verifier_env *env) verbose(env, "verifier bug. subprog has tail_call and async cb\n"); return -EFAULT; } - /* async callbacks don't increase bpf prog stack size */ - continue; + /* async callbacks don't increase bpf prog stack size unless called directly */ + if (!bpf_pseudo_call(insn + i)) + continue; } i = next_insn;