[CFT}[PATCH] coredump: Limit what can interrupt coredumps

[Eric W. Biederman]

Olivier Langlois has been struggling with coredumps written incompletely
in processes using io_uring.

Olivier Langlois <olivier@xxxxxxxxxxxxxx> writes:
> io_uring is a big user of task_work and any event that io_uring made a
> task waiting for that occurs during the core dump generation will
> generate a TIF_NOTIFY_SIGNAL.
>
> Here are the detailed steps of the problem:
> 1. io_uring calls vfs_poll() to install a task to a file wait queue
>    with io_async_wake() as the wakeup function cb from io_arm_poll_handler()
> 2. wakeup function ends up calling task_work_add() with TWA_SIGNAL
> 3. task_work_add() sets the TIF_NOTIFY_SIGNAL bit by calling
>    set_notify_signal()

The coredump code deliberately supports being interrupted by SIGKILL,
and depends upon prepare_signal to filter out all other signals.   Now
that signal_pending includes wake ups for TIF_NOTIFY_SIGNAL this hack
in dump_emitted by the coredump code no longer works.

Make the coredump code more robust by explicitly testing for all of
the wakeup conditions the coredump code supports.  This prevents
new wakeup conditions from breaking the coredump code, as well
as fixing the current issue.

The filesystem code that the coredump code uses already limits
itself to only aborting on fatal_signal_pending.  So it should
not develop surprising wake-up reasons either.

With dump_interrupted properly testing for the reasons it supports
being interrupted remove the special case from prepare_signal.

Fixes: 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL")
Reported-by: Olivier Langlois <olivier@xxxxxxxxxxxxxx>
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---

Olivier can you test this, and confirm this works for you?

 fs/coredump.c   | 2 +-
 kernel/signal.c | 2 --
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/coredump.c b/fs/coredump.c
index 2868e3e171ae..c3d8fc14b993 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -519,7 +519,7 @@ static bool dump_interrupted(void)
 	 * but then we need to teach dump_write() to restart and clear
 	 * TIF_SIGPENDING.
 	 */
-	return signal_pending(current);
+	return fatal_signal_pending(current) || freezing(current);
 }
 
 static void wait_for_dump_helpers(struct file *file)
diff --git a/kernel/signal.c b/kernel/signal.c
index f7c6ffcbd044..83d534deeb76 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -943,8 +943,6 @@ static bool prepare_signal(int sig, struct task_struct *p, bool force)
 	sigset_t flush;
 
 	if (signal->flags & (SIGNAL_GROUP_EXIT | SIGNAL_GROUP_COREDUMP)) {
-		if (!(signal->flags & SIGNAL_GROUP_EXIT))
-			return sig == SIGKILL;
 		/*
 		 * The process is in the middle of dying, nothing to do.
 		 */
-- 
2.20.1