On Fri, Jul 7, 2023 at 12:46 PM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
>
> On Sat, Jul 8, 2023 at 1:39 AM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote:
> >
> > On Wed, Jun 28, 2023 at 11:48:52AM +0100, David Howells wrote:
> > > Fscache has an optimisation by which reads from the cache are skipped until
> > > we know that (a) there's data there to be read and (b) that data isn't
> > > entirely covered by pages resident in the netfs pagecache. This is done
> > > with two flags manipulated by fscache_note_page_release():
> > >
> > > if (...
> > > test_bit(FSCACHE_COOKIE_HAVE_DATA, &cookie->flags) &&
> > > test_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags))
> > > clear_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
> > >
> > > where the NO_DATA_TO_READ flag causes cachefiles_prepare_read() to indicate
> > > that netfslib should download from the server or clear the page instead.
> > >
> > > The fscache_note_page_release() function is intended to be called from
> > > ->releasepage() - but that only gets called if PG_private or PG_private_2
> > > is set - and currently the former is at the discretion of the network
> > > filesystem and the latter is only set whilst a page is being written to the
> > > cache, so sometimes we miss clearing the optimisation.
> > >
> > > Fix this by following Willy's suggestion[1] and adding an address_space
> > > flag, AS_RELEASE_ALWAYS, that causes filemap_release_folio() to always call
> > > ->release_folio() if it's set, even if PG_private or PG_private_2 aren't
> > > set.
> > >
> > > Note that this would require folio_test_private() and page_has_private() to
> > > become more complicated. To avoid that, in the places[*] where these are
> > > used to conditionalise calls to filemap_release_folio() and
> > > try_to_release_page(), the tests are removed the those functions just
> > > jumped to unconditionally and the test is performed there.
> > >
> > > [*] There are some exceptions in vmscan.c where the check guards more than
> > > just a call to the releaser. I've added a function, folio_needs_release()
> > > to wrap all the checks for that.
> > >
> > > AS_RELEASE_ALWAYS should be set if a non-NULL cookie is obtained from
> > > fscache and cleared in ->evict_inode() before truncate_inode_pages_final()
> > > is called.
> > >
> > > Additionally, the FSCACHE_COOKIE_NO_DATA_TO_READ flag needs to be cleared
> > > and the optimisation cancelled if a cachefiles object already contains data
> > > when we open it.
> > >
> > > Fixes: 1f67e6d0b188 ("fscache: Provide a function to note the release of a page")
> > > Fixes: 047487c947e8 ("cachefiles: Implement the I/O routines")
> > > Reported-by: Rohith Surabattula <rohiths.msft@xxxxxxxxx>
> > > Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> >
> > Hi David,
> >
> > I was bisecting a use-after-free BUG on the latest mm-unstable,
> > where HEAD is 347e208de0e4 ("rmap: pass the folio to __page_check_anon_rmap()").
> >
> > According to my bisection, this is the first bad commit.
> > Use-After-Free is triggered on reclamation path when swap is enabled.
>
> This was originally occurred during kernel compilation but
> can easily be reproduced via:
>
> stress-ng --bigheap $(nproc)
>
> > (and couldn't trigger without swap enabled)
> >
> > the config, KASAN splat, bisect log are attached.
> > hope this isn't too late :(
> >
> > > cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > cc: Steve French <sfrench@xxxxxxxxx>
> > > cc: Shyam Prasad N <nspmangalore@xxxxxxxxx>
> > > cc: Rohith Surabattula <rohiths.msft@xxxxxxxxx>
> > > cc: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> > > cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> > > cc: Ilya Dryomov <idryomov@xxxxxxxxx>
> > > cc: linux-cachefs@xxxxxxxxxx
> > > cc: linux-cifs@xxxxxxxxxxxxxxx
> > > cc: linux-afs@xxxxxxxxxxxxxxxxxxx
> > > cc: v9fs-developer@xxxxxxxxxxxxxxxxxxxxx
> > > cc: ceph-devel@xxxxxxxxxxxxxxx
> > > cc: linux-nfs@xxxxxxxxxxxxxxx
> > > cc: linux-fsdevel@xxxxxxxxxxxxxxx
> > > cc: linux-mm@xxxxxxxxx
> > > ---
> > >
> > > Notes:
> > > ver #7)
> > > - Make NFS set AS_RELEASE_ALWAYS.
> > >
> > > ver #4)
> > > - Split out merging of folio_has_private()/filemap_release_folio() call
> > > pairs into a preceding patch.
> > > - Don't need to clear AS_RELEASE_ALWAYS in ->evict_inode().
> > >
> > > ver #3)
> > > - Fixed mapping_clear_release_always() to use clear_bit() not set_bit().
> > > - Moved a '&&' to the correct line.
> > >
> > > ver #2)
> > > - Rewrote entirely according to Willy's suggestion[1].
> > >
> > > fs/9p/cache.c | 2 ++
> > > fs/afs/internal.h | 2 ++
> > > fs/cachefiles/namei.c | 2 ++
> > > fs/ceph/cache.c | 2 ++
> > > fs/nfs/fscache.c | 3 +++
> > > fs/smb/client/fscache.c | 2 ++
> > > include/linux/pagemap.h | 16 ++++++++++++++++
> > > mm/internal.h | 5 ++++-
> > > 8 files changed, 33 insertions(+), 1 deletion(-)
I think myself / Daire Byrne may have already tracked this down and I
found a 1-liner that fixed a similar crash in his environment.
Can you try this patch on top and let me know if it still crashes?
https://github.com/DaveWysochanskiRH/kernel/commit/902c990e311120179fa5de99d68364b2947b79ec
