Re: [PATCH] kernel: make TIF_NOTIFY_SIGNAL and core dumps co-exist

[Olivier Langlois]

On Tue, 2021-08-17 at 21:06 -0600, Jens Axboe wrote:
> task_work being added with notify == TWA_SIGNAL will utilize
> TIF_NOTIFY_SIGNAL for signaling the targeted task that work is
> available.
> If this happens while a task is going through a core dump, it'll
> potentially disturb and truncate the dump as a signal interruption.
> 
> Have task_work_add() with notify == TWA_SIGNAL check if a task has
> been
> signaled for a core dump, and refuse to add the work if that is the
> case.
> When a core dump is invoked, explicitly check for TIF_NOTIFY_SIGNAL
> and
> run any pending task_work if that is set. This is similar to how an
> exiting task will not get new task_work added, and we return the same
> error for the core dump case. As we return success or failure from
> task_work_add(), the caller has to be prepared to handle this case
> already.
> 
> Currently this manifests itself in that io_uring tasks that end up
> using
> task_work will experience truncated core dumps.
> 
> Reported-by: Tony Battersby <tonyb@xxxxxxxxxxxxxxx>
> Reported-by: Olivier Langlois <olivier@xxxxxxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 5.10+
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> 
> ---
> 
> diff --git a/fs/coredump.c b/fs/coredump.c
> index 07afb5ddb1c4..ca7c1ee44ada 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -602,6 +602,14 @@ void do_coredump(const kernel_siginfo_t
> *siginfo)
>                 .mm_flags = mm->flags,
>         };
>  
> +       /*
> +        * task_work_add() will refuse to add work after PF_SIGNALED
> has
> +        * been set, ensure that we flush any pending
> TIF_NOTIFY_SIGNAL work
> +        * if any was queued before that.
> +        */
> +       if (test_thread_flag(TIF_NOTIFY_SIGNAL))
> +               tracehook_notify_signal();
> +
>         audit_core_dumps(siginfo->si_signo);
>  
>         binfmt = mm->binfmt;
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 1698fbe6f0e1..1ab28904adc4 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -41,6 +41,12 @@ int task_work_add(struct task_struct *task, struct
> callback_head *work,
>                 head = READ_ONCE(task->task_works);
>                 if (unlikely(head == &work_exited))
>                         return -ESRCH;
> +               /*
> +                * TIF_NOTIFY_SIGNAL notifications will interfere
> with
> +                * a core dump in progress, reject them.
> +                */
> +               if (notify == TWA_SIGNAL && (task->flags &
> PF_SIGNALED))
> +                       return -ESRCH;
>                 work->next = head;
>         } while (cmpxchg(&task->task_works, head, work) != head);
> 

tested successfully on 5.12.19

Tested-by: Olivier Langlois <olivier@xxxxxxxxxxxxxx>