Re: exec error: BUG: Bad rss-counter

[Eric W. Biederman]

Ilya Lipnitskiy <ilya.lipnitskiy@xxxxxxxxx> writes:

> 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
> possible that last year's exec changes have exposed this issue? Not
> blaming exec at all, just making sure I understand the problem better.

Only in the sense that copy_strings_kernel does less work than
"set_fs(KERNEL_DS); copy_strings; set_fs(USER_DS);"

Nothing huge was changed in exec but lots was moved around so that
it was clearer what is happening, and so that hacks like set_fs could
be removed.

Eric