[PATCH 2/2] nilfs2: protect access to buffers with no active references

[Ryusuke Konishi <konishi.ryusuke@gmail.com>]

nilfs_lookup_dirty_data_buffers(), which iterates through the buffers
attached to dirty data folios/pages, accesses the attached buffers
without locking the folios/pages.

For data cache, nilfs_clear_folio_dirty() may be called asynchronously
when the file system degenerates to read only, so
nilfs_lookup_dirty_data_buffers() still has the potential to cause use
after free issues when buffers lose the protection of their dirty state
midway due to this asynchronous clearing and are unintentionally freed
by try_to_free_buffers().

Eliminate this race issue by adjusting the lock section in this
function.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Fixes: 8c26c4e2694a ("nilfs2: fix issue with flush kernel thread after remount in RO mode because of driver's internal error or metadata corruption")
---
 fs/nilfs2/segment.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 587251830897..58a598b548fa 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -734,7 +734,6 @@ static size_t nilfs_lookup_dirty_data_buffers(struct inode *inode,
 		if (!head)
 			head = create_empty_buffers(folio,
 					i_blocksize(inode), 0);
-		folio_unlock(folio);
 
 		bh = head;
 		do {
@@ -744,11 +743,14 @@ static size_t nilfs_lookup_dirty_data_buffers(struct inode *inode,
 			list_add_tail(&bh->b_assoc_buffers, listp);
 			ndirties++;
 			if (unlikely(ndirties >= nlimit)) {
+				folio_unlock(folio);
 				folio_batch_release(&fbatch);
 				cond_resched();
 				return ndirties;
 			}
 		} while (bh = bh->b_this_page, bh != head);
+
+		folio_unlock(folio);
 	}
 	folio_batch_release(&fbatch);
 	cond_resched();
-- 
2.43.0