[PATCH] ocfs2: Fix potential ERR_PTR dereference in ocfs2_unlock_and_free_folios

[Zhiyu Zhang <zhiyuzhang999@gmail.com>]

When allocation of a folio fails (e.g., -ENOMEM), ocfs2_write_begin_nolock()
stores an ERR_PTR in wc->w_folios[i]. ocfs2_unlock_and_free_folios() later
walks this array but only skips NULL entries, thus passing the error pointer
to folio_unlock(), which eventually dereferences it in const_folio_flags and
panics with BUG: unable to handle kernel paging request in const_folio_flags.

This patch fixes this issue by adding IS_ERR_OR_NULL() to filter both NULL
and error entries before any folio operations.

Fixes: 7e119cff9d0a ("ocfs2: convert w_pages to w_folios")
Reported-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Closes: https://lore.kernel.org/linux-fsdevel/CALf2hKtnFskBvmZeigK_=mqq9Vd4TWT+YOXcwfNNt1ydOY=0Yg@mail.gmail.com/T/#u
Tested-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Signed-off-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
---
 fs/ocfs2/aops.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
index 40b6bce12951..9e500c5fee38 100644
--- a/fs/ocfs2/aops.c
+++ b/fs/ocfs2/aops.c
@@ -760,7 +760,7 @@ void ocfs2_unlock_and_free_folios(struct folio **folios, int num_folios)
 	int i;

 	for(i = 0; i < num_folios; i++) {
-		if (!folios[i])
+		if (IS_ERR_OR_NULL(folios[i]))
 			continue;
 		folio_unlock(folios[i]);
 		folio_mark_accessed(folios[i]);
--
2.34.1