Re: [PATCH v5] mm/gup: check page hwposion status for coredump.

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Fri, Mar 26, 2021 at 03:22:49PM +0100, David Hildenbrand wrote:
> On 26.03.21 15:09, David Hildenbrand wrote:
> > On 22.03.21 12:33, Aili Yao wrote:
> > > When we do coredump for user process signal, this may be one SIGBUS signal
> > > with BUS_MCEERR_AR or BUS_MCEERR_AO code, which means this signal is
> > > resulted from ECC memory fail like SRAR or SRAO, we expect the memory
> > > recovery work is finished correctly, then the get_dump_page() will not
> > > return the error page as its process pte is set invalid by
> > > memory_failure().
> > > 
> > > But memory_failure() may fail, and the process's related pte may not be
> > > correctly set invalid, for current code, we will return the poison page,
> > > get it dumped, and then lead to system panic as its in kernel code.
> > > 
> > > So check the hwpoison status in get_dump_page(), and if TRUE, return NULL.
> > > 
> > > There maybe other scenario that is also better to check hwposion status
> > > and not to panic, so make a wrapper for this check, Thanks to David's
> > > suggestion(<david@redhat.com>).
> > > 
> > > Link: https://lkml.kernel.org/r/20210319104437.6f30e80d@alex-virtual-machine
> > > Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
> > > Cc: David Hildenbrand <david@redhat.com>
> > > Cc: Matthew Wilcox <willy@infradead.org>
> > > Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > > Cc: Oscar Salvador <osalvador@suse.de>
> > > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > > Cc: Aili Yao <yaoaili@kingsoft.com>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > > ---
> > >    mm/gup.c      |  4 ++++
> > >    mm/internal.h | 20 ++++++++++++++++++++
> > >    2 files changed, 24 insertions(+)
> > > 
> > > diff --git a/mm/gup.c b/mm/gup.c
> > > index e4c224c..6f7e1aa 100644
> > > --- a/mm/gup.c
> > > +++ b/mm/gup.c
> > > @@ -1536,6 +1536,10 @@ struct page *get_dump_page(unsigned long addr)
> > >    				      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
> > >    	if (locked)
> > >    		mmap_read_unlock(mm);
> > 
> > Thinking again, wouldn't we get -EFAULT from __get_user_pages_locked()
> > when stumbling over a hwpoisoned page?
> > 
> > See __get_user_pages_locked()->__get_user_pages()->faultin_page():
> > 
> > handle_mm_fault()->vm_fault_to_errno(), which translates
> > VM_FAULT_HWPOISON to -EFAULT, unless FOLL_HWPOISON is set (-> -EHWPOISON)
> > 
> > ?

We could get -EFAULT, but sometimes not (depends on how memory_failure() fails).

If we failed to unmap, the page table is not converted to hwpoison entry,
so __get_user_pages_locked() get the hwpoisoned page.

If we successfully unmapped but failed in truncate_error_page() for example,
the processes mapping the page would get -EFAULT as expected.  But even in
this case, other processes could reach the error page via page cache and
__get_user_pages_locked() for them could return the hwpoisoned page.

> 
> Or doesn't that happen as you describe "But memory_failure() may fail, and
> the process's related pte may not be correctly set invalid" -- but why does
> that happen?

Simply because memory_failure() doesn't handle some page types like ksm page
and zero page. Or maybe shmem thp also belongs to this class.

> 
> On a similar thought, should get_user_pages() never return a page that has
> HWPoison set? E.g., check also for existing PTEs if the page is hwpoisoned?

Make sense to me. Maybe inserting hwpoison check into follow_page_pte() and
follow_huge_pmd() would work well.

Thanks,
Naoya Horiguchi