Date: Mon, 11 Mar 2024 15:48:14 +0100
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: Mirsad Todorovac <mirsad.todorovac@....unizg.hr>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
 Axel Rasmussen <axelrasmussen@...gle.com>
Subject: Re: BUG selftests/mm]

On 11.03.24 15:35, Peter Xu wrote:
> On Mon, Mar 11, 2024 at 10:31:41AM +0100, David Hildenbrand wrote:
>> On 09.03.24 20:12, Mirsad Todorovac wrote:
>>> Hi,
>>>
>>> Routine run of the test in net-next gave also this mm unit error.
>>>
>>> root@...iant:tools/testing/selftests/mm# ./uffd-unit-tests
>>> Testing UFFDIO_API (with syscall)... done
>>> Testing UFFDIO_API (with /dev/userfaultfd)... done
>>> Testing register-ioctls on anon... done
>>> Testing register-ioctls on shmem... done
>>> Testing register-ioctls on shmem-private... done
>>> Testing register-ioctls on hugetlb... skipped [reason: memory allocation failed]
>>> Testing register-ioctls on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing zeropage on anon... done
>>> Testing zeropage on shmem... done
>>> Testing zeropage on shmem-private... done
>>> Testing zeropage on hugetlb... skipped [reason: memory allocation failed]
>>> Testing zeropage on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing move on anon... done
>>> Testing move-pmd on anon... done
>>> Testing move-pmd-split on anon... done
>>> Testing wp-fork on anon... done
>>> Testing wp-fork on shmem... done
>>> Testing wp-fork on shmem-private... done
>>> Testing wp-fork on hugetlb... skipped [reason: memory allocation failed]
>>> Testing wp-fork on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing wp-fork-with-event on anon... done
>>> Testing wp-fork-with-event on shmem... done
>>> Testing wp-fork-with-event on shmem-private... done
>>> Testing wp-fork-with-event on hugetlb... skipped [reason: memory allocation failed]
>>> Testing wp-fork-with-event on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing wp-fork-pin on anon... done
>>> Testing wp-fork-pin on shmem... done
>>> Testing wp-fork-pin on shmem-private... done
>>> Testing wp-fork-pin on hugetlb... skipped [reason: memory allocation failed]
>>> Testing wp-fork-pin on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing wp-fork-pin-with-event on anon... done
>>> Testing wp-fork-pin-with-event on shmem... done
>>> Testing wp-fork-pin-with-event on shmem-private... done
>>> Testing wp-fork-pin-with-event on hugetlb... skipped [reason: memory allocation failed]
>>> Testing wp-fork-pin-with-event on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing wp-unpopulated on anon... done
>>> Testing minor on shmem... done
>>> Testing minor on hugetlb... skipped [reason: memory allocation failed]
>>> Testing minor-wp on shmem... done
>>> Testing minor-wp on hugetlb... skipped [reason: memory allocation failed]
>>> Testing minor-collapse on shmem... done
>>> Testing sigbus on anon... done
>>> Testing sigbus on shmem... done
>>> Testing sigbus on shmem-private... done
>>> Testing sigbus on hugetlb... skipped [reason: memory allocation failed]
>>> Testing sigbus on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing sigbus-wp on anon... done
>>> Testing sigbus-wp on shmem... done
>>> Testing sigbus-wp on shmem-private... done
>>> Testing sigbus-wp on hugetlb... skipped [reason: memory allocation failed]
>>> Testing sigbus-wp on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing events on anon... done
>>> Testing events on shmem... done
>>> Testing events on shmem-private... done
>>> Testing events on hugetlb... skipped [reason: memory allocation failed]
>>> Testing events on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing events-wp on anon... done
>>> Testing events-wp on shmem... done
>>> Testing events-wp on shmem-private... done
>>> Testing events-wp on hugetlb... skipped [reason: memory allocation failed]
>>> Testing events-wp on hugetlb-private... skipped [reason: memory allocation failed]
>>> Testing poison on anon... done
>>> Testing poison on shmem... done
>>> Testing poison on shmem-private... done
>>> Testing poison on hugetlb... skipped [reason: memory allocation failed]
>>> Testing poison on hugetlb-private... skipped [reason: memory allocation failed]
>>> Userfaults unit tests: pass=42, skip=24, fail=0 (total=66)
>>> root@...iant:tools/testing/selftests/mm# grep -i huge /proc/meminfo
>>>
>>> It resulted in alarming errors in the syslog:
>>>
>>> Mar  9 19:48:24 defiant kernel: [77187.055103] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4631e000
>>> Mar  9 19:48:24 defiant kernel: [77187.055132] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46320000
>>> Mar  9 19:48:24 defiant kernel: [77187.055160] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46322000
>>> Mar  9 19:48:24 defiant kernel: [77187.055189] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46324000
>>> Mar  9 19:48:24 defiant kernel: [77187.055218] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46326000
>>> Mar  9 19:48:24 defiant kernel: [77187.055250] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46328000
>>> Mar  9 19:48:24 defiant kernel: [77187.055278] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4632a000
>>> Mar  9 19:48:24 defiant kernel: [77187.055307] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4632c000
>>> Mar  9 19:48:24 defiant kernel: [77187.055336] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4632e000
>>> Mar  9 19:48:24 defiant kernel: [77187.055366] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46330000
>>> Mar  9 19:48:24 defiant kernel: [77187.055395] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46332000
>>> Mar  9 19:48:24 defiant kernel: [77187.055423] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46334000
>>> Mar  9 19:48:24 defiant kernel: [77187.055452] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46336000
>>> Mar  9 19:48:24 defiant kernel: [77187.055480] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46338000
>>> Mar  9 19:48:24 defiant kernel: [77187.055509] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4633a000
>>> Mar  9 19:48:24 defiant kernel: [77187.055538] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4633c000
>>> Mar  9 19:48:24 defiant kernel: [77187.055567] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 4633e000
>>> Mar  9 19:48:24 defiant kernel: [77187.055597] MCE: Killing uffd-unit-tests:1321817 due to hardware memory corruption fault at 46340000
>>>
>>> At this point, it can be problem with my box's memory chips, or something with HUGETLB.
>>>
>>> However, since the "classic" allocations were successful, the problem might be in huge pages, or
>>> if I understood well, in deliberate poisoning of pages?
>>>
>>
>> Isn't that just the (expected) side effect of UFFDIO_POISON tests?
>>
>> IOW, there is no problem here. We are poisoning virtual memory locations
>> (not actual memory) and expect a SIGBUS on next access. While testing that,
>> we receive these messages.
> 
> Correct.
> 
>>
>> The "ugly" thing here seems to be that we can trigger repeated pr_err() from
>> user space. There is no rate-limiting in place. Maybe UFFDIO_POISON requires
>> root permissions so this cannot be exploited by unprivileged user space to
>> flood the system log?
>>
>> CCing Axel
> 
> This is pretty unfortunate.
> 
> I'm not concerned too much on flooding whoever kicks off the selftests, but
> indeed this seems to be able to be used by anyone to trigger such endless
> reports in dmesg.

Right.

> 
> The issue with requiring a privilege means any hypervisor that will need to
> use this to emulate memory errors will also require such privilege, and it
> can be a problem.
> 

Yes, we don't want that.

> Logically such "hwpoison errors" are not real so it is not needed to be
> reported in dmesg, but now we're leveraging it to be exactly the same as a
> real hw error to share the code path, iiuc (e.g. on MCE injections).
> 
> One option is to use a different marker reflecting that such hwpoison error
> is internal, so we don't need to report in dmesg. That'll also require
> (besides another bit in pte markers) one extra VM_FAULT_* flag just for
> such reports.  Might be slightly an overkill, but I don't see another
> better way; not reporting HWPOISON will complicate at least kvm use case
> even more.
> 
> Or.. does syslog has its own protection in general for such printk floods?
> It'll be easier if that's not a concern to flood then, but I'm not sure
> from that regard.

 From what I know, flooding is considered problematic and we fix it up 
using "Fixes:" commits. See 1b0a151c10a6d823f033023b9fdd9af72a89591b as 
one "recent" example.


Usually we switch to the _ratelimited() functions, maybe 
pr_warn_ratelimited() is good enough? But we'd lose some details on a 
"real" MCE storm, though.

-- 
Cheers,

David / dhildenb

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