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Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
From: Olivier Langlois <olivier@trillion01.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
io-uring <io-uring@vger.kernel.org>,
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"Pavel Begunkov>" <asml.silence@gmail.com>,
Oleg Nesterov <oleg@redhat.com>
Date: Sat, 12 Jun 2021 10:36:15 -0400
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On Thu, 2021-06-10 at 15:11 -0500, Eric W. Biederman wrote:
>
> Olivier Langlois has been struggling with coredumps being incompletely
> written in
> processes using io_uring.
>
> Olivier Langlois <olivier@trillion01.com> 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.
>
> v2: Don't remove the now unnecessary code in prepare_signal.
>
> Cc: stable@vger.kernel.org
> Fixes: 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL")
> Reported-by: Olivier Langlois <olivier@trillion01.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
> fs/coredump.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> 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)
Tested-by: Olivier Langlois <olivier@trillion01.com>