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)) {
+ 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);
--
2.43.0
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