Message-ID: <CALCv0x2-Q9o7k1jhzN73nZ9F5+tcp7T8SkLKQWXW=1gLLJNegA@mail.gmail.com>
Date:   Tue, 2 Mar 2021 23:01:16 -0800
From:   Ilya Lipnitskiy <ilya.lipnitskiy@...il.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: exec error: BUG: Bad rss-counter

On Tue, Mar 2, 2021 at 11:37 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> Ilya Lipnitskiy <ilya.lipnitskiy@...il.com> writes:
>
> > On Mon, Mar 1, 2021 at 12:43 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
> >>
> >> Ilya Lipnitskiy <ilya.lipnitskiy@...il.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.

>
> > Here is the (optimized) stack trace when the counter does not get decremented:
> > [<8015b078>] vm_normal_page+0x114/0x1a8
> > [<8015dc98>] unmap_page_range+0x388/0xacc
> > [<8015e5a0>] unmap_vmas+0x6c/0x98
> > [<80166194>] exit_mmap+0xd8/0x1ac
> > [<800290c0>] mmput+0x58/0xf8
> > [<801a6f8c>] free_bprm+0x2c/0xc4
> > [<801a8890>] kernel_execve+0x160/0x1d8
> > [<800420e0>] call_usermodehelper_exec_async+0x114/0x194
> > [<80003198>] ret_from_kernel_thread+0x14/0x1c
> >
> >>
> >> AKA it looks very very fishy this happens and this does not look like
> >> an execve error.
> > I think you are right, I'm probably wrong to bother you. However,
> > since the thread is already started, let me add linux-mm here :)
>
> It happens during exec.  I don't mind looking and pointing you a useful
> direction.
>
> >>
> >> On the other hand it would be good to know why kernel_execve is failing.
> >> Then the error handling paths could be scrutinized, and we can check to
> >> see if everything that should happen on an error path does.
> > I can check on this, but likely it's the init system not doing things
> > quite in the right order on my platform, or something similar. The
> > error is ENOENT from do_open_execat().
>
> That does narrow things down considerably.
> After the error all we do is:
> Clear in_execve and fs->in_exec.
> Return from bprm_execve
> Call free_bprm
> Which does:
>         if (bprm->mm) {
>                 acct_arg_size(bprm, 0);
>                 mmput(bprm->mm);
>         }
>
> So it really needs to be the mmput that cleans things up.\
>
> I would really verify the correspondence between what get_arg_page
> returns and what gets freed in mmput if it is not too difficult.
> I think it should just be a page or two.
>
> Eric

Ilya

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