Re: exec error: BUG: Bad rss-counter

[Ilya Lipnitskiy]

On Sat, Mar 20, 2021 at 8:59 AM Zhou Yanjie <zhouyanjie@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
> >> Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> writes:
> >>
> >>> On Tue, Mar 2, 2021 at 11:37 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >>>> Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> writes:
> >>>>
> >>>>> On Mon, Mar 1, 2021 at 12:43 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> >>>>>> Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> 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);

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