[PATCH 2/4] coredump: prevent TIF_NOTIFY_SIGNAL from interrupting coredumps

TIF_NOTIFY_SIGNAL is used to kick a task in uninterruptible sleep to
return to userspace and run task work and then go back to sleep. This
mechanism works well but breaks coredumps. dump_interrupted() only
allows fatal signals to interrupt a coredump and the whole regular write
path going to actual filesystems is impervious to TIF_NOTIFY_SIGNAL as
well.

However, both the usermodehelper pipe and the coredump socket will bail
early on TIF_NOTIFY_SIGNAL. This affects the following codepaths:

- coredump_sock_recv() -> unix_stream_read_generic()
  The request/ack handshake is abandoned before any coredump data is
  sent.

- anon_pipe_write() returning -ERESTARTSYS
  Once the pipe is full this truncates the coredump.

- unix_stream_sendmsg() returning -ERESTARTSYS
  Once the send buffer is full this truncates the coredump.

- coredump_sock_wait() -> __kernel_read()
  This reports a failure that didn't happen.

- wait_for_dump_helpers() -> wait_event_interruptible()
  This stops waiting for the coredump helpers.

Such truncation is entirely invisible to userspace and all uapi bits
still indicate that a successful coredump happened. A crashing process
with a bunch of file backed mappings and io_uring thrown in loses most
of the coredump data. If the NT_FILE note goes past PAGE_SIZE mappings
it's gonzo.

TIF_NOTIFY_SIGNAL is sent by io_uring for the common case. And it uses
poll without sleeping so a completion callback runs task_work_add() from
interrupt context against the task that submitted the request. This is
the task that is running the coredump. Since that task hasn't set
work_exited (it hasn't exited yet after all) TIF_NOTIFY_SIGNAL keeps
reappearing.

A coredumping task doesn't return to userspace. The task work is run at
exit. So interrupting it doesn't buy anything and just loses the
coredump which is quite valuable.

Note that this isn't specific to io_uring. There's also
klp_send_signals(), bpf_task_work_schedule_signal(), landlock's tsync
and then  technically, kthread_stop() and the printk kunit test set the
bit raw.

Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
 fs/coredump.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/coredump.c b/fs/coredump.c
index e68a76ff92a3..edb5a1a61d84 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -1165,6 +1165,7 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	struct core_name cn;
 	const struct mm_struct *mm = current->mm;
 	const struct linux_binfmt *binfmt = mm->binfmt;
+	unsigned int notify_flags;
 	int argc = 0;
 	struct coredump_params cprm = {
 		.siginfo = siginfo,
@@ -1196,9 +1197,12 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
 	if (coredump_wait(siginfo->si_signo, &core_state) < 0)
 		return;
 
+	/* Task work must not cut the dump short, see signal_pending(). */
+	notify_flags = no_notify_signal_save();
 	scoped_with_creds(cred)
 		do_coredump(&cn, &cprm, &argv, &argc, binfmt);
 	coredump_cleanup(&cn, &cprm);
+	no_notify_signal_restore(notify_flags);
 	return;
 }
 

-- 
2.53.0