bpf: Fix a possible task gone issue with bpf_send_signal[_thread]() helpers

In current bpf_send_signal() and bpf_send_signal_thread() helper implementation, irq_work is used to handle nmi context. Hao Sun reported in [1] that the current task at the entry of the helper might be gone during irq_work callback processing. To fix the issue, a reference is acquired for the current task before enqueuing into the irq_work so that the queued task is still available during irq_work callback processing.

[1] https://lore.kernel.org/bpf/[email protected]/

---
 kernel/trace/bpf_trace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

NOTE:
I didn't add a unit test case since it is very hard to construct one which can reliably reproducing the issue in short amount of time. I cannot even reproduce the issue with Hao's reproducer in my local environment. Hopefully, the patch itself can explain the issue and the fix.

index f47274de012b..c09792c551bf 100644
@@ -833,6 +833,7 @@ static void do_bpf_send_signal(struct irq_work *entry)
 
 	work = container_of(entry, struct send_signal_irq_work, irq_work);
 	group_send_sig_info(work->sig, SEND_SIG_PRIV, work->task, work->type);
+	put_task_struct(work->task);
 }
 
 static int bpf_send_signal_common(u32 sig, enum pid_type type)

@@ -867,7 +868,7 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type)
 		 * to the irq_work. The current task may change when queued
 		 * irq works get executed.
 		 */
-		work->task = current;
+		work->task = get_task_struct(current);
 		work->sig = sig;
 		work->type = type;
 		irq_work_queue(&work->irq_work);

2.30.2