[PATCH bpf v1 07/10] bpf: Mark sched_process_wait argument as nullable

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

do_wait() passes wo->wo_pid to the sched_process_wait tracepoint.
kernel_wait4() leaves wo_pid NULL for wait4(-1), and
kernel_waitid_prepare() does likewise for waitid(P_ALL).

btf_ctx_access() currently types argument 0 as PTR_TO_BTF_ID |
PTR_TRUSTED. Without PTR_MAYBE_NULL, the verifier accepts an unchecked
dereference. Trusted pointer loads have no fault protection, so a wait for
any child can then cause a NULL pointer dereference in JITed BPF code.

Add sched_process_wait to raw_tp_null_args[] with argument 0 marked
nullable. The verifier rejects an unchecked dereference while preserving
access after the program checks the pointer for NULL.

Fixes: 838a10bd2ebf ("bpf: Augment raw_tp arguments with PTR_MAYBE_NULL")
Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/btf.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index d6d243c262ea..41764356d2d3 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6657,6 +6657,10 @@ struct bpf_raw_tp_null_args {
 static const struct bpf_raw_tp_null_args raw_tp_null_args[] = {
 	/* sched */
 	{ "sched_pi_setprio", 0x10 },
+	/*
+	 * do_wait() passes NULL for wait4(-1) and waitid(P_ALL).
+	 */
+	{ "sched_process_wait", 0x1 },
 	/* ... from sched_numa_pair_template event class */
 	{ "sched_stick_numa", 0x100 },
 	{ "sched_swap_numa", 0x100 },
-- 
2.53.0