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Subject: Re: exec error: BUG: Bad rss-counter
To:     Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Christoph Hellwig <hch@lst.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
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From:   Zhou Yanjie <zhouyanjie@wanyeetech.com>
Message-ID: <05dc921e-da71-4e71-6132-736eccd35680@wanyeetech.com>
Date:   Tue, 30 Mar 2021 12:56:07 +0800
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Hi Ilya,

On 2021/3/29 上午10:48, Ilya Lipnitskiy wrote:
> On Sat, Mar 20, 2021 at 8:59 AM Zhou Yanjie <zhouyanjie@wanyeetech.com> wrote:
>> Hi Ilya,
>>
>> On 2021/3/3 下午11:55, Ilya Lipnitskiy wrote:
>>> On Wed, Mar 3, 2021 at 7:50 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>> Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com> writes:
>>>>
>>>>> On Tue, Mar 2, 2021 at 11:37 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>>>> Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com> writes:
>>>>>>
>>>>>>> On Mon, Mar 1, 2021 at 12:43 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>>>>>> Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com> writes:
>>>>>>>>
>>>>>>>>> Eric, All,
>>>>>>>>>
>>>>>>>>> The following error appears when running Linux 5.10.18 on an embedded
>>>>>>>>> MIPS mt7621 target:
>>>>>>>>> [    0.301219] BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:1
>>>>>>>>>
>>>>>>>>> Being a very generic error, I started digging and added a stack dump
>>>>>>>>> before the BUG:
>>>>>>>>> Call Trace:
>>>>>>>>> [<80008094>] show_stack+0x30/0x100
>>>>>>>>> [<8033b238>] dump_stack+0xac/0xe8
>>>>>>>>> [<800285e8>] __mmdrop+0x98/0x1d0
>>>>>>>>> [<801a6de8>] free_bprm+0x44/0x118
>>>>>>>>> [<801a86a8>] kernel_execve+0x160/0x1d8
>>>>>>>>> [<800420f4>] call_usermodehelper_exec_async+0x114/0x194
>>>>>>>>> [<80003198>] ret_from_kernel_thread+0x14/0x1c
>>>>>>>>>
>>>>>>>>> So that's how I got to looking at fs/exec.c and noticed quite a few
>>>>>>>>> changes last year. Turns out this message only occurs once very early
>>>>>>>>> at boot during the very first call to kernel_execve. current->mm is
>>>>>>>>> NULL at this stage, so acct_arg_size() is effectively a no-op.
>>>>>>>> If you believe this is a new error you could bisect the kernel
>>>>>>>> to see which change introduced the behavior you are seeing.
>>>>>>>>
>>>>>>>>> More digging, and I traced the RSS counter increment to:
>>>>>>>>> [<8015adb4>] add_mm_counter_fast+0xb4/0xc0
>>>>>>>>> [<80160d58>] handle_mm_fault+0x6e4/0xea0
>>>>>>>>> [<80158aa4>] __get_user_pages.part.78+0x190/0x37c
>>>>>>>>> [<8015992c>] __get_user_pages_remote+0x128/0x360
>>>>>>>>> [<801a6d9c>] get_arg_page+0x34/0xa0
>>>>>>>>> [<801a7394>] copy_string_kernel+0x194/0x2a4
>>>>>>>>> [<801a880c>] kernel_execve+0x11c/0x298
>>>>>>>>> [<800420f4>] call_usermodehelper_exec_async+0x114/0x194
>>>>>>>>> [<80003198>] ret_from_kernel_thread+0x14/0x1c
>>>>>>>>>
>>>>>>>>> In fact, I also checked vma_pages(bprm->vma) and lo and behold it is set to 1.
>>>>>>>>>
>>>>>>>>> How is fs/exec.c supposed to handle implied RSS increments that happen
>>>>>>>>> due to page faults when discarding the bprm structure? In this case,
>>>>>>>>> the bug-generating kernel_execve call never succeeded, it returned -2,
>>>>>>>>> but I didn't trace exactly what failed.
>>>>>>>> Unless I am mistaken any left over pages should be purged by exit_mmap
>>>>>>>> which is called by mmput before mmput calls mmdrop.
>>>>>>> Good to know. Some more digging and I can say that we hit this error
>>>>>>> when trying to unmap PFN 0 (is_zero_pfn(pfn) returns TRUE,
>>>>>>> vm_normal_page returns NULL, zap_pte_range does not decrement
>>>>>>> MM_ANONPAGES RSS counter). Is my understanding correct that PFN 0 is
>>>>>>> usable, but special? Or am I totally off the mark here?
>>>>>> It would be good to know if that is the page that get_user_pages_remote
>>>>>> returned to copy_string_kernel.  The zero page that is always zero,
>>>>>> should never be returned when a writable mapping is desired.
>>>>> Indeed, pfn 0 is returned from get_arg_page: (page is 0x809cf000,
>>>>> page_to_pfn(page) is 0) and it is the same page that is being freed and not
>>>>> refcounted in mmput/zap_pte_range. Confirmed with good old printk. Also,
>>>>> ZERO_PAGE(0)==0x809fc000 -> PFN 5120.
>>>>>
>>>>> I think I have found the problem though, after much digging and thanks to all
>>>>> the information provided. init_zero_pfn() gets called too late (after
>>>>> the call to
>>>>> is_zero_pfn(0) from mmput returns true), until then zero_pfn == 0, and after,
>>>>> zero_pfn == 5120. Boom.
>>>>>
>>>>> So PFN 0 is special, but only for a little bit, enough for something
>>>>> on my system
>>>>> to call kernel_execve :)
>>>>>
>>>>> Question: is my system not supposed to be calling kernel_execve this
>>>>> early or does
>>>>> init_zero_pfn() need to happen earlier? init_zero_pfn is currently a
>>>>> core_initcall.
>>>> Looking quickly it seems that init_zero_pfn() is in mm/memory.c and is
>>>> common for both mips and x86.  Further it appears init_zero_pfn() has
>>>> been that was since 2009 a13ea5b75964 ("mm: reinstate ZERO_PAGE").
>>>>
>>>> Given the testing that x86 gets and that nothing like this has been
>>>> reported it looks like whatever driver is triggering the kernel_execve
>>>> is doing something wrong.
>>>> Because honestly.  If the zero page isn't working there is not a chance
>>>> that anything in userspace is working so it is clearly much too early.
>>>>
>>>> I suspect there is some driver that is initialized very early that is
>>>> doing something that looks innocuous (like triggering a hotplug event)
>>>> and that happens to cause a call_usermode_helper which then calls
>>>> kernel_execve.
>>> I will investigate the offenders more closely. However, I do not
>>> notice this behavior on the same system based on the 5.4 kernel. Is it
>>
>> I also encountered this problem on Ingenic X1000 and X1830. This is the
>> printed information:
>>
>> [    0.120715] BUG: Bad rss-counter state mm:(ptrval)
>>    type:MM_ANONPAGES val:1
>>
>> I tested kernel 5.9, kernel 5.10, kernel 5.11, and kernel 5.12, only
>> kernel 5.9 did not have this problem, so we can know that this problem
>> was introduced in kernel 5.10, have you found any effective solution?
> Try:
> diff --git a/mm/memory.c b/mm/memory.c
> index c8e357627318..1fd753245369 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -166,7 +166,7 @@ static int __init init_zero_pfn(void)
>          zero_pfn = page_to_pfn(ZERO_PAGE(0));
>          return 0;
>   }
> -core_initcall(init_zero_pfn);
> +early_initcall(init_zero_pfn);


It works, thanks!


Best regards!


>   void mm_trace_rss_stat(struct mm_struct *mm, int member, long count)
>   {