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

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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. A concurrent caller 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.

kprobe, tracepoint and perf_event programs exclude each other on
the same CPU through bpf_prog_active, so for two callers to meet,
one of the programs has to be a raw tracepoint or an fentry one.
And since commit 87c544108b ("bpf: Send signals asynchronously
if !preemptible") this path runs with IRQs enabled too, so a hard
IRQ can interrupt the fill as well, not only an NMI. Found by code
inspection and reproduced with a perf_event NMI program racing an
fentry one.

Claim the work through work->task itself: cmpxchg() it from NULL
to the target task before touching the other fields, and set it
back to NULL once the callback has consumed them. A context
finding the work claimed returns the documented -EBUSY.

Fixes: 1bc7896e9e ("bpf: Fix deadlock with rq_lock in bpf_send_signal()")
Signed-off-by: Yuan Chen <chenyuan@kylinos.cn>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261009022427.968304-1-chenyuan@kylinos.cn
This commit is contained in:
Yuan Chen 2026-10-09 10:24:27 +08:00 committed by Alexei Starovoitov
parent d2141481a7
commit 2d1f101ee6
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1 changed files with 5 additions and 2 deletions
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7
kernel/trace/bpf_trace.c
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@ -823,6 +823,7 @@ const struct bpf_func_proto bpf_task_pt_regs_proto = {
struct send_signal_irq_work {
struct irq_work irq_work;
/* The claim: NULL marks the slot free for the next caller. */
struct task_struct *task;
u32 sig;
enum pid_type type;
@ -842,6 +843,8 @@ 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. */
smp_store_release(&work->task, NULL);
}
static int bpf_send_signal_common(u32 sig, enum pid_type type, struct task_struct *task, u64 value)
@ -885,14 +888,14 @@ 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 (cmpxchg(&work->task, NULL, task))
return -EBUSY;
/* Add the current task, which is the target of sending signal,
* to the irq_work. The current task may change when queued
* irq works get executed.
*/
work->task = get_task_struct(task);
get_task_struct(task);
work->has_siginfo = siginfo == &info;
if (work->has_siginfo)
copy_siginfo(&work->info, &info);