[PATCH 6.12.y 1/2] fork: move the coredump and exec checks into create_io_thread()

[Sasha Levin <sashal@kernel.org>]

From: Christian Brauner <brauner@kernel.org>

[ Upstream commit e760200370e4de054b6b5f568a7faa911e2956bb ]

Only a PF_USER_WORKER thread returns from get_signal() after a fatal
signal and can go on to create a thread. get_signal() sets PF_SIGNALED
before it dumps core and before it lets a PF_USER_WORKER thread return.
That flags sticks.

The exec'ing thread itself is never signaled. But it doesn't need the check
anyway as we moved cancellation of io_uring requests before de_thread().
create_io_thread() is the only way a signaled thread creates another
one. vhost_task_create() is reached from ioctls alone.

io_should_retry_thread() doesn't retry -EINTR, so io-wq callers give up
the same way they did when copy_process() refused.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/aq0VLTyxNatFmLOK@redhat.com
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-13-8e4adb1619e6@kernel.org
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Stable-dep-of: 1f978881ce6c ("fork: don't create io threads once PF_POSTCOREDUMP is set")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/fork.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index f22b79d9954a8..c82956f8fc9d5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2774,6 +2774,10 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node)
 		.user_worker	= 1,
 	};
 
+	/* A creator past its fatal signal gets no thread. */
+	if (current->flags & PF_SIGNALED)
+		return ERR_PTR(-EINTR);
+
 	return copy_process(NULL, 0, node, &args);
 }
 
-- 
2.53.0