Re: [PATCH v2] secretmem: fix unhandled fault in truncate

[Jan Kara <jack@suse.cz>]

On Tue 12-07-22 10:40:11, Yang Shi wrote:
> On Fri, Jul 8, 2022 at 1:29 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > On Thu, Jul 07, 2022 at 03:09:32PM -0700, Yang Shi wrote:
> > > On Thu, Jul 7, 2022 at 1:55 PM Darrick J. Wong <djwong@kernel.org> wrote:
> > > >
> > > > On Thu, Jul 07, 2022 at 10:48:00AM -0700, Yang Shi wrote:
> > > > > On Thu, Jul 7, 2022 at 9:57 AM Mike Rapoport <rppt@kernel.org> wrote:
> > > > > >
> > > > > > Eric Biggers suggested that this happens when
> > > > > > secretmem_setattr()->simple_setattr() races with secretmem_fault() so
> > > > > > that a page that is faulted in by secretmem_fault() (and thus removed
> > > > > > from the direct map) is zeroed by inode truncation right afterwards.
> > > > > >
> > > > > > Since do_truncate() takes inode_lock(), adding inode_lock_shared() to
> > > > > > secretmem_fault() prevents the race.
> > > > >
> > > > > Should invalidate_lock be used to serialize between page fault and truncate?
> > > >
> > > > I would have thought so, given Documentation/filesystems/locking.rst:
> > > >
> > > > "->fault() is called when a previously not present pte is about to be
> > > > faulted in. The filesystem must find and return the page associated with
> > > > the passed in "pgoff" in the vm_fault structure. If it is possible that
> > > > the page may be truncated and/or invalidated, then the filesystem must
> > > > lock invalidate_lock, then ensure the page is not already truncated
> > > > (invalidate_lock will block subsequent truncate), and then return with
> > > > VM_FAULT_LOCKED, and the page locked. The VM will unlock the page."
> > > >
> > > > IIRC page faults aren't supposed to take i_rwsem because the fault could
> > > > be in response to someone mmaping a file into memory and then write()ing
> > > > to the same file using the mmapped region.  The write() takes
> > > > inode_lock and faults on the buffer, so the fault cannot take inode_lock
> > > > again.
> > >
> > > Do you mean writing from one part of the file to the other part of the
> > > file so the "from" buffer used by copy_from_user() is part of the
> > > mmaped region?
> > >
> > > Another possible deadlock issue by using inode_lock in page faults is
> > > mmap_lock is acquired before inode_lock, but write may acquire
> > > inode_lock before mmap_lock, it is a AB-BA lock pattern, but it should
> > > not cause real deadlock since mmap_lock is not exclusive for page
> > > faults. But such pattern should be avoided IMHO.
> > >
> > > > That said... I don't think memfd_secret files /can/ be written to?
> >
> > memfd_secret files cannot be written to, they can only be mmap()ed.
> > Synchronization is only required between
> > do_truncate()->...->simple_setatt() and secretmem->fault() and I don't see
> > how that can deadlock.
> 
> Sure, there is no deadlock.
> 
> >
> > I'm not an fs expert though, so if you think that invalidate_lock() is
> > safer, I don't mind s/inode_lock/invalidate_lock/ in the patch.
> 
> IIUC invalidate_lock should be preferred per the filesystem's locking
> document. And I found Jan Kara's email of the invalidate_lock
> patchset, please refer to
> https://lore.kernel.org/linux-mm/20210715133202.5975-1-jack@suse.cz/.

Yeah, so using invalidate_lock for such synchronization would be certainly
more standard than using inode_lock. Although I agree that for filesystems
that do not support read(2) and write(2) there does not seem to be an
immediate risk of a deadlock when inode_lock is used inside a page fault.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR