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Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
To:     Jens Axboe <axboe@kernel.dk>,
        Olivier Langlois <olivier@trillion01.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        "Pavel Begunkov>" <asml.silence@gmail.com>
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 <87eeda7nqe.fsf@disp2133>
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 <87pmwt6biw.fsf@disp2133> <87czst5yxh.fsf_-_@disp2133>
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 <87y2bh4jg5.fsf@disp2133>
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 <87sg1p4h0g.fsf_-_@disp2133> <20210614141032.GA13677@redhat.com>
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From:   Tony Battersby <tonyb@cybernetics.com>
Message-ID: <3168284a-0b52-7845-07b1-a72bdfed915c@cybernetics.com>
Date:   Tue, 17 Aug 2021 17:39:18 -0400
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On 8/17/21 5:28 PM, Jens Axboe wrote:
>
> Another approach - don't allow TWA_SIGNAL task_work to get queued if
> PF_SIGNALED has been set on the task. This is similar to how we reject
> task_work_add() on process exit, and the callers must be able to handle
> that already.
>
> Can you test this one on top of your 5.10-stable?
>
>
> 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 ((task->flags & PF_SIGNALED) && notify == TWA_SIGNAL)
> +			return -ESRCH;
>  		work->next = head;
>  	} while (cmpxchg(&task->task_works, head, work) != head);
>  
>
Doesn't compile.  5.10 doesn't have TIF_NOTIFY_SIGNAL.

Tony Battersby