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

[Jens Axboe <axboe@kernel.dk>]

On 8/17/21 1:59 PM, Jens Axboe wrote:
> On 8/17/21 1:29 PM, Tony Battersby wrote:
>> On 8/17/21 2:24 PM, Jens Axboe wrote:
>>> On 8/17/21 12:15 PM, Jens Axboe wrote:
>>>> On 8/15/21 2:42 PM, Olivier Langlois wrote:
>>>>> On Wed, 2021-08-11 at 19:55 -0600, Jens Axboe wrote:
>>>>>> On 8/10/21 3:48 PM, Tony Battersby wrote:
>>>>>>> On 8/5/21 9:06 AM, Olivier Langlois wrote:
>>>>>>>> Hi all,
>>>>>>>>
>>>>>>>> I didn't forgot about this remaining issue and I have kept thinking
>>>>>>>> about it on and off.
>>>>>>>>
>>>>>>>> I did try the following on 5.12.19:
>>>>>>>>
>>>>>>>> diff --git a/fs/coredump.c b/fs/coredump.c
>>>>>>>> index 07afb5ddb1c4..614fe7a54c1a 100644
>>>>>>>> --- a/fs/coredump.c
>>>>>>>> +++ b/fs/coredump.c
>>>>>>>> @@ -41,6 +41,7 @@
>>>>>>>>  #include <linux/fs.h>
>>>>>>>>  #include <linux/path.h>
>>>>>>>>  #include <linux/timekeeping.h>
>>>>>>>> +#include <linux/io_uring.h>
>>>>>>>>  
>>>>>>>>  #include <linux/uaccess.h>
>>>>>>>>  #include <asm/mmu_context.h>
>>>>>>>> @@ -625,6 +626,8 @@ void do_coredump(const kernel_siginfo_t
>>>>>>>> *siginfo)
>>>>>>>>                 need_suid_safe = true;
>>>>>>>>         }
>>>>>>>>  
>>>>>>>> +       io_uring_files_cancel(current->files);
>>>>>>>> +
>>>>>>>>         retval = coredump_wait(siginfo->si_signo, &core_state);
>>>>>>>>         if (retval < 0)
>>>>>>>>                 goto fail_creds;
>>>>>>>> --
>>>>>>>> 2.32.0
>>>>>>>>
>>>>>>>> with my current understanding, io_uring_files_cancel is supposed to
>>>>>>>> cancel everything that might set the TIF_NOTIFY_SIGNAL.
>>>>>>>>
>>>>>>>> I must report that in my testing with generating a core dump
>>>>>>>> through a
>>>>>>>> pipe with the modif above, I still get truncated core dumps.
>>>>>>>>
>>>>>>>> systemd is having a weird error:
>>>>>>>> [ 2577.870742] systemd-coredump[4056]: Failed to get COMM: No such
>>>>>>>> process
>>>>>>>>
>>>>>>>> and nothing is captured
>>>>>>>>
>>>>>>>> so I have replaced it with a very simple shell:
>>>>>>>> $ cat /proc/sys/kernel/core_pattern 
>>>>>>>>> /home/lano1106/bin/pipe_core.sh %e %p
>>>>>>>> ~/bin $ cat pipe_core.sh 
>>>>>>>> #!/bin/sh
>>>>>>>>
>>>>>>>> cat > /home/lano1106/core/core.$1.$2
>>>>>>>>
>>>>>>>> BFD: warning: /home/lano1106/core/core.test.10886 is truncated:
>>>>>>>> expected core file size >= 24129536, found: 61440
>>>>>>>>
>>>>>>>> I conclude from my attempt that maybe io_uring_files_cancel is not
>>>>>>>> 100%
>>>>>>>> cleaning everything that it should clean.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>> I just ran into this problem also - coredumps from an io_uring
>>>>>>> program
>>>>>>> to a pipe are truncated.  But I am using kernel 5.10.57, which does
>>>>>>> NOT
>>>>>>> have commit 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL")
>>>>>>> or
>>>>>>> commit 06af8679449d ("coredump: Limit what can interrupt coredumps").
>>>>>>> Kernel 5.4 works though, so I bisected the problem to commit
>>>>>>> f38c7e3abfba ("io_uring: ensure async buffered read-retry is setup
>>>>>>> properly") in kernel 5.9.  Note that my io_uring program uses only
>>>>>>> async
>>>>>>> buffered reads, which may be why this particular commit makes a
>>>>>>> difference to my program.
>>>>>>>
>>>>>>> My io_uring program is a multi-purpose long-running program with many
>>>>>>> threads.  Most threads don't use io_uring but a few of them do. 
>>>>>>> Normally, my core dumps are piped to a program so that they can be
>>>>>>> compressed before being written to disk, but I can also test writing
>>>>>>> the
>>>>>>> core dumps directly to disk.  This is what I have found:
>>>>>>>
>>>>>>> *) Unpatched 5.10.57: if a thread that doesn't use io_uring triggers
>>>>>>> a
>>>>>>> coredump, the core file is written correctly, whether it is written
>>>>>>> to
>>>>>>> disk or piped to a program, even if another thread is using io_uring
>>>>>>> at
>>>>>>> the same time.
>>>>>>>
>>>>>>> *) Unpatched 5.10.57: if a thread that uses io_uring triggers a
>>>>>>> coredump, the core file is truncated, whether written directly to
>>>>>>> disk
>>>>>>> or piped to a program.
>>>>>>>
>>>>>>> *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring
>>>>>>> triggers a coredump, and the core is written directly to disk, then
>>>>>>> it
>>>>>>> is written correctly.
>>>>>>>
>>>>>>> *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring
>>>>>>> triggers a coredump, and the core is piped to a program, then it is
>>>>>>> truncated.
>>>>>>>
>>>>>>> *) 5.10.57+revert f38c7e3abfba: core dumps are written correctly,
>>>>>>> whether written directly to disk or piped to a program.
>>>>>> That is very interesting. Like Olivier mentioned, it's not that actual
>>>>>> commit, but rather the change of behavior implemented by it. Before
>>>>>> that
>>>>>> commit, we'd hit the async workers more often, whereas after we do the
>>>>>> correct retry method where it's driven by the wakeup when the page is
>>>>>> unlocked. This is purely speculation, but perhaps the fact that the
>>>>>> process changes state potentially mid dump is why the dump ends up
>>>>>> being
>>>>>> truncated?
>>>>>>
>>>>>> I'd love to dive into this and try and figure it out. Absent a test
>>>>>> case, at least the above gives me an idea of what to try out. I'll see
>>>>>> if it makes it easier for me to create a case that does result in a
>>>>>> truncated core dump.
>>>>>>
>>>>> Jens,
>>>>>
>>>>> When I have first encountered the issue, the very first thing that I
>>>>> did try was to create a simple test program that would synthetize the
>>>>> problem.
>>>>>
>>>>> After few time consumming failed attempts, I just gave up the idea and
>>>>> simply settle to my prod program that showcase systematically the
>>>>> problem every time that I kill the process with a SEGV signal.
>>>>>
>>>>> In a nutshell, all the program does is to issue read operations with
>>>>> io_uring on a TCP socket on which there is a constant data stream.
>>>>>
>>>>> Now that I have a better understanding of what is going on, I think
>>>>> that one way that could reproduce the problem consistently could be
>>>>> along those lines:
>>>>>
>>>>> 1. Create a pipe
>>>>> 2. fork a child
>>>>> 3. Initiate a read operation on the pipe with io_uring from the child
>>>>> 4. Let the parent kill its child with a core dump generating signal.
>>>>> 5. Write something in the pipe from the parent so that the io_uring
>>>>> read operation completes while the core dump is generated.
>>>>>
>>>>> I guess that I'll end up doing that if I cannot fix the issue with my
>>>>> current setup but here is what I have attempted so far:
>>>>>
>>>>> 1. Call io_uring_files_cancel from do_coredump
>>>>> 2. Same as #1 but also make sure that TIF_NOTIFY_SIGNAL is cleared on
>>>>> returning from io_uring_files_cancel
>>>>>
>>>>> Those attempts didn't work but lurking in the io_uring dev mailing list
>>>>> is starting to pay off. I thought that I did reach the bottom of the
>>>>> rabbit hole in my journey of understanding io_uring but the recent
>>>>> patch set sent by Hao Xu
>>>>>
>>>>> https://lore.kernel.org/io-uring/90fce498-968e-6812-7b6a-fdf8520ea8d9@kernel.dk/T/#t
>>>>>
>>>>> made me realize that I still haven't assimilated all the small io_uring
>>>>> nuances...
>>>>>
>>>>> Here is my feedback. From my casual io_uring code reader point of view,
>>>>> it is not 100% obvious what the difference is between
>>>>> io_uring_files_cancel and io_uring_task_cancel
>>>>>
>>>>> It seems like io_uring_files_cancel is cancelling polls only if they
>>>>> have the REQ_F_INFLIGHT flag set.
>>>>>
>>>>> I have no idea what an inflight request means and why someone would
>>>>> want to call io_uring_files_cancel over io_uring_task_cancel.
>>>>>
>>>>> I guess that if I was to meditate on the question for few hours, I
>>>>> would at some point get some illumination strike me but I believe that
>>>>> it could be a good idea to document in the code those concepts for
>>>>> helping casual readers...
>>>>>
>>>>> Bottomline, I now understand that io_uring_files_cancel does not cancel
>>>>> all the requests. Therefore, without fully understanding what I am
>>>>> doing, I am going to replace my call to io_uring_files_cancel from
>>>>> do_coredump with io_uring_task_cancel and see if this finally fix the
>>>>> issue for good.
>>>>>
>>>>> What I am trying to do is to cancel pending io_uring requests to make
>>>>> sure that TIF_NOTIFY_SIGNAL isn't set while core dump is generated.
>>>>>
>>>>> Maybe another solution would simply be to modify __dump_emit to make it
>>>>> resilient to TIF_NOTIFY_SIGNAL as Eric W. Biederman originally
>>>>> suggested.
>>>>>
>>>>> or maybe do both...
>>>>>
>>>>> Not sure which approach is best. If someone has an opinion, I would be
>>>>> curious to hear it.
>>>> It does indeed sound like it's TIF_NOTIFY_SIGNAL that will trigger some
>>>> signal_pending() and cause an interruption of the core dump. Just out of
>>>> curiosity, what is your /proc/sys/kernel/core_pattern set to? If it's
>>>> set to some piped process, can you try and set it to 'core' and see if
>>>> that eliminates the truncation of the core dumps for your case?
>>> And assuming that works, then I suspect this one would fix your issue
>>> even with a piped core dump:
>>>
>>> diff --git a/fs/coredump.c b/fs/coredump.c
>>> index 07afb5ddb1c4..852737a9ccbf 100644
>>> --- a/fs/coredump.c
>>> +++ b/fs/coredump.c
>>> @@ -41,6 +41,7 @@
>>>  #include <linux/fs.h>
>>>  #include <linux/path.h>
>>>  #include <linux/timekeeping.h>
>>> +#include <linux/io_uring.h>
>>>  
>>>  #include <linux/uaccess.h>
>>>  #include <asm/mmu_context.h>
>>> @@ -603,6 +604,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>>>  	};
>>>  
>>>  	audit_core_dumps(siginfo->si_signo);
>>> +	io_uring_task_cancel();
>>>  
>>>  	binfmt = mm->binfmt;
>>>  	if (!binfmt || !binfmt->core_dump)
>>>
>> FYI, I tested kernel 5.10.59 + backport 06af8679449d + the patch above
>> with my io_uring program.  The coredump locked up even when writing the
>> core file directly to disk; the zombie process could not be killed with
>> "kill -9".  Unfortunately I can't test with newer kernels without
>> spending some time on it, and I am too busy with other stuff right now.
> 
> That sounds like 5.10-stable is missing some of the cancelation
> backports, and your setup makes the cancelation stall because of that.
> Need to go over the 11/12/13 fixes and ensure that we've got everything
> we need for those stable versions, particularly 5.10.
> 
>> My io_uring program does async buffered reads
>> (io_uring_prep_read()/io_uring_prep_readv()) from a raw disk partition
>> (no filesystem).  One thread submits I/Os while another thread calls
>> io_uring_wait_cqe() and processes the completions.  To trigger the
>> coredump, I added an intentional abort() in the thread that submits I/Os
>> after running for a second.
> 
> OK, so that one is also using task_work for the retry based async
> buffered reads, so it makes sense.
> 
> Maybe a temporary work-around is to use 06af8679449d and eliminate the
> pipe based coredump?

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);
 

-- 
Jens Axboe