Re: [PATCH 3/3] s390/mm: fix mis-accounting of pgtable_bytes

[Martin Schwidefsky]

On Wed, 31 Oct 2018 14:43:38 +0800
Li Wang <liwang@xxxxxxxxxx> wrote:

> On Wed, Oct 31, 2018 at 2:31 PM, Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> wrote:
> 
> > On Wed, 31 Oct 2018 14:18:33 +0800
> > Li Wang <liwang@xxxxxxxxxx> wrote:
> >  
> > > On Tue, Oct 16, 2018 at 12:42 AM, Martin Schwidefsky <  
> > schwidefsky@xxxxxxxxxx  
> > > > wrote:  
> > >  
> > > > In case a fork or a clone system fails in copy_process and the error
> > > > handling does the mmput() at the bad_fork_cleanup_mm label, the
> > > > following warning messages will appear on the console:
> > > >
> > > >   BUG: non-zero pgtables_bytes on freeing mm: 16384
> > > >
> > > > The reason for that is the tricks we play with mm_inc_nr_puds() and
> > > > mm_inc_nr_pmds() in init_new_context().
> > > >
> > > > A normal 64-bit process has 3 levels of page table, the p4d level and
> > > > the pud level are folded. On process termination the free_pud_range()
> > > > function in mm/memory.c will subtract 16KB from pgtable_bytes with a
> > > > mm_dec_nr_puds() call, but there actually is not really a pud table.
> > > >
> > > > One issue with this is the fact that pgtable_bytes is usually off
> > > > by a few kilobytes, but the more severe problem is that for a failed
> > > > fork or clone the free_pgtables() function is not called. In this case
> > > > there is no mm_dec_nr_puds() or mm_dec_nr_pmds() that go together with
> > > > the mm_inc_nr_puds() and mm_inc_nr_pmds in init_new_context().
> > > > The pgtable_bytes will be off by 16384 or 32768 bytes and we get the
> > > > BUG message. The message itself is purely cosmetic, but annoying.
> > > >
> > > > To fix this override the mm_pmd_folded, mm_pud_folded and mm_p4d_folded
> > > > function to check for the true size of the address space.
> > > >  
> > >
> > > I can confirm that it works to the problem, the warning message is gone
> > > after applying this patch on s390x. And I also done ltp syscalls/cve test
> > > for the patch set on x86_64 arch, there has no new regression.
> > >
> > > Tested-by: Li Wang <liwang@xxxxxxxxxx>  
> >
> > Thanks for testing. Unfortunately Heiko reported another issue yesterday
> > with the patch applied. This time the other way around:
> >
> > BUG: non-zero pgtables_bytes on freeing mm: -16384
> >  
> 
> Okay, the problem is still triggered by LTP/cve-2017-17052.c?

No, unfortunately we do not have a simple testcase to trigger this new bug.
It happened once with one of our test kernels, the path that leads to this
is completely unclear.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.