[PATCH] bpf: Claim the per-CPU send_signal irq_work before filling it

[chenyuan_fl@163.com]

From: Yuan Chen <chenyuan@kylinos.cn>

irq_work_is_busy() cannot see the per-CPU send_signal_work while
it is being filled: the check only matches after irq_work_queue()
has claimed the work. An NMI interrupting the fill therefore passes
it, both callers race for the same irq_work, and the loser's signal
is silently lost along with its task reference while the queued
work runs with a mix of both callers' fields.

Claim the work with an atomic gate before touching any of its
fields and release it only after the callback has consumed them. A
context finding the work claimed returns the documented -EBUSY,
and the return value of irq_work_queue() is now handled.

Fixes: 1bc7896e9ef4 ("bpf: Fix deadlock with rq_lock in bpf_send_signal()")
Signed-off-by: Yuan Chen <chenyuan@kylinos.cn>
---
 kernel/trace/bpf_trace.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 891897f8a1b3..03552637aab9 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -823,6 +823,8 @@ const struct bpf_func_proto bpf_task_pt_regs_proto = {
 
 struct send_signal_irq_work {
 	struct irq_work irq_work;
+	/* Covers the fill-to-run span which irq_work_is_busy() cannot see. */
+	atomic_t claimed;
 	struct task_struct *task;
 	u32 sig;
 	enum pid_type type;
@@ -842,6 +844,8 @@ static void do_bpf_send_signal(struct irq_work *entry)
 
 	group_send_sig_info(work->sig, siginfo, work->task, work->type);
 	put_task_struct(work->task);
+	/* Release once the fields are consumed. */
+	atomic_set_release(&work->claimed, 0);
 }
 
 static int bpf_send_signal_common(u32 sig, enum pid_type type, struct task_struct *task, u64 value)
@@ -885,7 +889,7 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type, struct task_struc
 			return -EINVAL;
 
 		work = this_cpu_ptr(&send_signal_work);
-		if (irq_work_is_busy(&work->irq_work))
+		if (atomic_xchg(&work->claimed, 1))
 			return -EBUSY;
 
 		/* Add the current task, which is the target of sending signal,
@@ -898,7 +902,12 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type, struct task_struc
 			copy_siginfo(&work->info, &info);
 		work->sig = sig;
 		work->type = type;
-		irq_work_queue(&work->irq_work);
+		if (unlikely(!irq_work_queue(&work->irq_work))) {
+			/* Unreachable while the claim is held. */
+			put_task_struct(task);
+			atomic_set_release(&work->claimed, 0);
+			return -EBUSY;
+		}
 		return 0;
 	}
 
-- 
2.54.0