Re: [PATCH] coredump: Limit what can interrupt coredumps

[Olivier Langlois]

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@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.
> 
> v2: Don't remove the now unnecessary code in prepare_signal.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL")
> Reported-by: Olivier Langlois <olivier@xxxxxxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
>  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@xxxxxxxxxxxxxx>