On Sat, Jul 8, 2023 at 1:39 AM Hyeonggon Yoo <42.hye...@gmail.com> 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.m...@gmail.com>
> > Suggested-by: Matthew Wilcox <wi...@infradead.org>
> > Signed-off-by: David Howells <dhowe...@redhat.com>
>
> 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 <wi...@infradead.org>
> > cc: Linus Torvalds <torva...@linux-foundation.org>
> > cc: Steve French <sfre...@samba.org>
> > cc: Shyam Prasad N <nspmangal...@gmail.com>
> > cc: Rohith Surabattula <rohiths.m...@gmail.com>
> > cc: Dave Wysochanski <dwyso...@redhat.com>
> > cc: Dominique Martinet <asmad...@codewreck.org>
> > cc: Ilya Dryomov <idryo...@gmail.com>
> > cc: linux-cachefs@redhat.com
> > cc: linux-c...@vger.kernel.org
> > cc: linux-...@lists.infradead.org
> > cc: v9fs-develo...@lists.sourceforge.net
> > cc: ceph-de...@vger.kernel.org
> > cc: linux-...@vger.kernel.org
> > cc: linux-fsde...@vger.kernel.org
> > cc: linux...@kvack.org
> > ---
> >
> > 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(-)
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