Re: [PATCH v7] f2fs: use post-decrement count for cp_wait wakeup

[Wenjie Qi]

  The UAF scenario described in v1/v2 does not look valid. The wakeup is
  already before folio_end_writeback(), and I do not see enough basis to
  claim that sbi->cp_wait can be freed at that point.

  After your comments, v3-v7 drifted to a different issue: using the
  post-decrement count to avoid missing the zero transition when another
  F2FS_WB_CP_DATA writeback is submitted before the second get_pages()
  check. That is not the same issue as the v1/v2 UAF description.

  Please drop this patch. If you think the wakeup-latency issue is
still worth fixing
  separately, I can send a new patch without the UAF wording, Fixes
tag, or stable Cc.

  Thanks,

On Sat, Jun 20, 2026 at 3:57 PM Chao Yu <chao@xxxxxxxxxx> wrote:
>
> On 6/18/26 18:05, Wenjie Qi wrote:
> > f2fs_write_end_io() decrements the writeback page counter and then reads
> > it again with get_pages() to decide whether the last F2FS_WB_CP_DATA
> > completion should wake cp_wait.
> >
> > That second read can miss the zero transition as below:
>
> Looks comments of v7 patch is quite different from the one of v1 patch?
>
> Quoted from v1:
>
> "f2fs_write_end_io() currently decrements the writeback page counter before
> waking sbi->cp_wait for the last F2FS_WB_CP_DATA completion.
>
> That decrement can drop the F2FS_WB_CP_DATA count to zero. It can unblock
> a concurrent unmount path waiting in f2fs_wait_on_all_pages(). Unmount can
> continue through f2fs_put_super() and eventually free sbi while the end_io
> callback is still about to evaluate wq_has_sleeper() and wake_up() on
> sbi->cp_wait.
>
> Commit 2d9c4a4ed4ee ("f2fs: fix UAF caused by decrementing sbi->nr_pages[]
> in f2fs_write_end_io()") fixed one post-decrement sbi access by moving the
> warm-node-list handling before dec_page_count(). The compressed writeback
> path follows the same rule and documents that dec_page_count() must be the
> last access to sbi when it can drop F2FS_WB_CP_DATA to zero.
>
> Apply the same ordering rule to the cp_wait wakeup. Check whether this is
> the last F2FS_WB_CP_DATA completion and wake the waiter before the counter
> decrement. Then the callback no longer dereferences sbi->cp_wait after the
> lifetime boundary. A waiter that runs before the decrement may observe old
> count and sleep until the one-jiffy timeout, but correctness no longer
> depends on touching sbi after the counter reaches zero."
>
> I may found something interesting: v7 codes try to fix UAF bug described in
> v1 comment, however v7 comment tries to explain what v2 codes want to do.
>
> I suspect your LLM goes another direction after prompted w/ my comments on
> patch v1? Let me know I'm wrong. :P
>
> Thanks,
>
> >
> > checkpoint          end_io A              submitter B
> > - f2fs_wait_on_all_pages
> >   - get_pages() > 0
> >   - prepare_to_wait(cp_wait)
> >   - io_schedule_timeout
> >                      - f2fs_write_end_io
> >                       - dec_page_count
> >                        : count 1 -> 0
> >                                           - f2fs_submit_page_write
> >                                            - inc_page_count
> >                                             : count 0 -> 1
> >                       - get_pages() > 0
> >                         : skip wake_up(cp_wait)
> >
> > The checkpoint thread can then keep sleeping until
> > DEFAULT_SCHEDULE_TIMEOUT, even though end_io A completed the old last
> > F2FS_WB_CP_DATA page.
> >
> > Use the post-decrement value for F2FS_WB_CP_DATA completions so the wakeup
> > decision is tied to this completion.  Keep the existing dec_page_count()
> > path for other writeback counters.
> >
> > Fixes: e234088758fc ("f2fs: avoid wait if IO end up when do_checkpoint for better performance")
> > Fixes: ce2739e482bc ("f2fs: fix to avoid UAF in f2fs_write_end_io()")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Wenjie Qi <qiwenjie@xxxxxxxxxx>
> > ---
> >   fs/f2fs/data.c | 12 +++++++-----
> >   fs/f2fs/f2fs.h |  6 ++++++
> >   2 files changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index d83a21998ec2..2afdcd209d54 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -392,15 +392,17 @@ static void f2fs_write_end_io(struct bio *bio)
> >               if (f2fs_in_warm_node_list(folio))
> >                       f2fs_del_fsync_node_entry(sbi, folio);
> >
> > -             dec_page_count(sbi, type);
> > -
> >               /*
> >                * we should access sbi before folio_end_writeback() to
> >                * avoid racing w/ kill_f2fs_super()
> >                */
> > -             if (type == F2FS_WB_CP_DATA && !get_pages(sbi, type) &&
> > -                             wq_has_sleeper(&sbi->cp_wait))
> > -                     wake_up(&sbi->cp_wait);
> > +             if (type == F2FS_WB_CP_DATA) {
> > +                     if (!dec_page_count_return(sbi, type) &&
> > +                         wq_has_sleeper(&sbi->cp_wait))
> > +                             wake_up(&sbi->cp_wait);
> > +             } else {
> > +                     dec_page_count(sbi, type);
> > +             }
> >
> >               folio_clear_f2fs_gcing(folio);
> >               folio_end_writeback(folio);
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 9f24287de4c3..db750cef371d 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -2776,6 +2776,12 @@ static inline void dec_page_count(struct f2fs_sb_info *sbi, int count_type)
> >       atomic_dec(&sbi->nr_pages[count_type]);
> >   }
> >
> > +static inline int dec_page_count_return(struct f2fs_sb_info *sbi,
> > +                                     int count_type)
> > +{
> > +     return atomic_dec_return(&sbi->nr_pages[count_type]);
> > +}
> > +
> >   static inline void inode_dec_dirty_pages(struct inode *inode)
> >   {
> >       if (!S_ISDIR(inode->i_mode) && !S_ISREG(inode->i_mode) &&
> >
> > base-commit: c0b65f6129c7fbb526e921dd60261650f1b2bef9
>