Re: [PATCH] mm: Avoid data corruption on CoW fault into PFN-mapped VMA

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Feb 19, 2020 at 01:22:39PM -0800, Andrew Morton wrote:
> On Tue, 18 Feb 2020 18:41:51 +0300 "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
> 
> > Jeff Moyer has reported that one of xfstests triggers a warning when run
> > on DAX-enabled filesystem:
> > 
> > 	WARNING: CPU: 76 PID: 51024 at mm/memory.c:2317 wp_page_copy+0xc40/0xd50
> > 	...
> > 	wp_page_copy+0x98c/0xd50 (unreliable)
> > 	do_wp_page+0xd8/0xad0
> > 	__handle_mm_fault+0x748/0x1b90
> > 	handle_mm_fault+0x120/0x1f0
> > 	__do_page_fault+0x240/0xd70
> > 	do_page_fault+0x38/0xd0
> > 	handle_page_fault+0x10/0x30
> > 
> > The warning happens on failed __copy_from_user_inatomic() which tries to
> > copy data into a CoW page.
> > 
> > This happens because of race between MADV_DONTNEED and CoW page fault:
> > 
> > 	CPU0					CPU1
> >  handle_mm_fault()
> >    do_wp_page()
> >      wp_page_copy()
> >        do_wp_page()
> > 					madvise(MADV_DONTNEED)
> > 					  zap_page_range()
> > 					    zap_pte_range()
> > 					      ptep_get_and_clear_full()
> > 					      <TLB flush>
> > 	 __copy_from_user_inatomic()
> > 	 sees empty PTE and fails
> > 	 WARN_ON_ONCE(1)
> > 	 clear_page()
> > 
> > The solution is to re-try __copy_from_user_inatomic() under PTL after
> > checking that PTE is matches the orig_pte.
> > 
> > The second copy attempt can still fail, like due to non-readable PTE,
> > but there's nothing reasonable we can do about, except clearing the CoW
> > page.
> 
> You don't think this is worthy of a cc:stable?

Please, add it.

Although, if I read history correctly, it is 15 year old bug that nobody
noticed until we added WARN() there :/

-- 
 Kirill A. Shutemov