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/20230109074425.12556-1-sunhao.th@gmail.com/

Fixes: 8b401f9e ("bpf: implement bpf_send_signal() helper")
Tested-by: Hao Sun <sunhao.th@gmail.com>
Reported-by: Hao Sun <sunhao.th@gmail.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230118204815.3331855-1-yhs@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/trace/bpf_trace.c

@@ -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);