nilfs2: fix potential bug in end_buffer_async_write

commit 5bc09b397cbf1221f8a8aacb1152650c9195b02b upstream.

According to a syzbot report, end_buffer_async_write(), which handles the
completion of block device writes, may detect abnormal condition of the
buffer async_write flag and cause a BUG_ON failure when using nilfs2.

Nilfs2 itself does not use end_buffer_async_write().  But, the async_write
flag is now used as a marker by commit 7f42ec394156 ("nilfs2: fix issue
with race condition of competition between segments for dirty blocks") as
a means of resolving double list insertion of dirty blocks in
nilfs_lookup_dirty_data_buffers() and nilfs_lookup_node_buffers() and the
resulting crash.

This modification is safe as long as it is used for file data and b-tree
node blocks where the page caches are independent.  However, it was
irrelevant and redundant to also introduce async_write for segment summary
and super root blocks that share buffers with the backing device.  This
led to the possibility that the BUG_ON check in end_buffer_async_write
would fail as described above, if independent writebacks of the backing
device occurred in parallel.

The use of async_write for segment summary buffers has already been
removed in a previous change.

Fix this issue by removing the manipulation of the async_write flag for
the remaining super root block buffer.

Link: https://lkml.kernel.org/r/20240203161645.4992-1-konishi.ryusuke@gmail.com
Fixes: 7f42ec394156 ("nilfs2: fix issue with race condition of competition between segments for dirty blocks")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+5c04210f7c7f897c1e7f@syzkaller.appspotmail.com
Closes: https://lkml.kernel.org/r/00000000000019a97c05fd42f8c8@google.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 418055ac910b6595e330ec8e3f86e1647ef540c6..be0ca35b8aa4b296b3f50c45757bdaa7c271e877 100644 (file)
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -1707,7 +1707,6 @@ static void nilfs_segctor_prepare_write(struct nilfs_sc_info *sci)
 
                list_for_each_entry(bh, &segbuf->sb_payload_buffers,
                                    b_assoc_buffers) {
-                       set_buffer_async_write(bh);
                        if (bh == segbuf->sb_super_root) {
                                if (bh->b_page != bd_page) {
                                        lock_page(bd_page);
@@ -1718,6 +1717,7 @@ static void nilfs_segctor_prepare_write(struct nilfs_sc_info *sci)
                                }
                                break;
                        }
+                       set_buffer_async_write(bh);
                        if (bh->b_page != fs_page) {
                                nilfs_begin_page_io(fs_page);
                                fs_page = bh->b_page;
@@ -1803,7 +1803,6 @@ static void nilfs_abort_logs(struct list_head *logs, int err)
 
                list_for_each_entry(bh, &segbuf->sb_payload_buffers,
                                    b_assoc_buffers) {
-                       clear_buffer_async_write(bh);
                        if (bh == segbuf->sb_super_root) {
                                clear_buffer_uptodate(bh);
                                if (bh->b_page != bd_page) {
@@ -1812,6 +1811,7 @@ static void nilfs_abort_logs(struct list_head *logs, int err)
                                }
                                break;
                        }
+                       clear_buffer_async_write(bh);
                        if (bh->b_page != fs_page) {
                                nilfs_end_page_io(fs_page, err);
                                fs_page = bh->b_page;
@@ -1899,8 +1899,9 @@ static void nilfs_segctor_complete_write(struct nilfs_sc_info *sci)
                                 BIT(BH_Delay) | BIT(BH_NILFS_Volatile) |
                                 BIT(BH_NILFS_Redirected));
 
-                       set_mask_bits(&bh->b_state, clear_bits, set_bits);
                        if (bh == segbuf->sb_super_root) {
+                               set_buffer_uptodate(bh);
+                               clear_buffer_dirty(bh);
                                if (bh->b_page != bd_page) {
                                        end_page_writeback(bd_page);
                                        bd_page = bh->b_page;
@@ -1908,6 +1909,7 @@ static void nilfs_segctor_complete_write(struct nilfs_sc_info *sci)
                                update_sr = true;
                                break;
                        }
+                       set_mask_bits(&bh->b_state, clear_bits, set_bits);
                        if (bh->b_page != fs_page) {
                                nilfs_end_page_io(fs_page, 0);
                                fs_page = bh->b_page;