On 5/26/26 11:44, Wenjie Qi wrote:
> f2fs_write_end_io() 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
> then continue through f2fs_put_super() and 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
> sbi accesses must happen before dec_page_count() can drop the
> F2FS_WB_CP_DATA count to zero.
>
> Use atomic_dec_and_lock_irqsave() for F2FS_WB_CP_DATA completions so the
> zero transition is serialized with cp_wait.lock. When the count reaches
> zero, wake waiters while holding the same lock.
>
> In f2fs_wait_on_all_pages(), prepare the waiter and recheck the page count
> while holding cp_wait.lock before sleeping. This keeps the wakeup visible
> to waiters without requiring the end_io callback to access sbi after the
> F2FS_WB_CP_DATA count has reached zero. It also avoids a missed wakeup that
> would otherwise make the waiter sleep until DEFAULT_SCHEDULE_TIMEOUT.
>
> Fixes: ce2739e482bc ("f2fs: fix to avoid UAF in f2fs_write_end_io()")
> Cc: [email protected]
> Signed-off-by: Wenjie Qi <[email protected]>
> ---
> fs/f2fs/checkpoint.c | 20 ++++++++++++++++++--
> fs/f2fs/data.c | 25 +++++++++++++++++--------
> 2 files changed, 35 insertions(+), 10 deletions(-)
>
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index c00a6b6ebcbd..b16d2d30ec6a 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1497,24 +1497,40 @@ static void unblock_operations(struct f2fs_sb_info
> *sbi)
> f2fs_unlock_all(sbi);
> }
>
> +static bool f2fs_prepare_cp_wait(struct f2fs_sb_info *sbi,
> + struct wait_queue_entry *wait, int type)
> +{
> + unsigned long flags;
> + bool wait_more;
> +
> + prepare_to_wait(&sbi->cp_wait, wait, TASK_UNINTERRUPTIBLE);
> + spin_lock_irqsave(&sbi->cp_wait.lock, flags);
> + wait_more = get_pages(sbi, type);
> + spin_unlock_irqrestore(&sbi->cp_wait.lock, flags);
> +
> + return wait_more;
> +}
> +
> void f2fs_wait_on_all_pages(struct f2fs_sb_info *sbi, int type)
> {
> DEFINE_WAIT(wait);
>
> for (;;) {
> - if (!get_pages(sbi, type))
> + if (!f2fs_prepare_cp_wait(sbi, &wait, type))
> break;
>
> if (unlikely(f2fs_cp_error(sbi) &&
> !is_sbi_flag_set(sbi, SBI_IS_CLOSE)))
> break;
> + finish_wait(&sbi->cp_wait, &wait);
>
> if (type == F2FS_DIRTY_META)
> f2fs_sync_meta_pages(sbi, LONG_MAX, FS_CP_META_IO);
> else if (type == F2FS_WB_CP_DATA)
> f2fs_submit_merged_write(sbi, DATA);
>
> - prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
> + if (!f2fs_prepare_cp_wait(sbi, &wait, type))
> + break;
> io_schedule_timeout(DEFAULT_SCHEDULE_TIMEOUT);
> }
> finish_wait(&sbi->cp_wait, &wait);
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index d83a21998ec2..d92f0b70ba2f 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -392,15 +392,24 @@ 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);
> + if (type == F2FS_WB_CP_DATA) {
> + unsigned long flags;
>
> - /*
> - * 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);
> + /*
> + * Hold cp_wait.lock across the zero transition and the
> + * wakeup so f2fs_wait_on_all_pages() cannot miss it or
> + * free sbi before this callback stops touching cp_wait.
> + */
> + if (atomic_dec_and_lock_irqsave(&sbi->nr_pages[type],
> + &sbi->cp_wait.lock,
> + flags)) {
if (atomic_dec_return(&sbi->nr_pages[type]) &&
wq_has_sleeper(&sbi->cp_wait)）
wake_up(&sbi->cp_wait);
Is it enough to solve the issue?
Thanks,
> + wake_up_locked(&sbi->cp_wait);
> + spin_unlock_irqrestore(&sbi->cp_wait.lock,
> + flags);
> + }
> + } else {
> + dec_page_count(sbi, type);
> + }
>
> folio_clear_f2fs_gcing(folio);
> folio_end_writeback(folio);
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