[PATCH] tmpfs: zero unused folio tail for long symlinks

[Yousef Alhouseen <alhouseenyousef@gmail.com>]

shmem_symlink() marks the entire folio uptodate after copying only the
NUL-terminated link target. The remainder of the freshly allocated folio
is left uninitialized.

Reclaim may pass the whole folio to a swap compressor. KMSAN observed
sw842_compress() computing a checksum over the uninitialized tail. If
the folio is written to a swap device, those bytes can also leave the
kernel.

Zero the remainder of the folio before marking it uptodate and dirty.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+bf5586280a66e9ccdfa9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bf5586280a66e9ccdfa9
Cc: stable@vger.kernel.org
Signed-off-by: Yousef Alhouseen <alhouseenyousef@gmail.com>
---
 mm/shmem.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index b51f83c970bb..b06c1ae2f50c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -4057,6 +4057,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
 			goto out_remove_offset;
 		inode->i_op = &shmem_symlink_inode_operations;
 		memcpy(folio_address(folio), symname, len);
+		folio_zero_range(folio, len, folio_size(folio) - len);
 		folio_mark_uptodate(folio);
 		folio_mark_dirty(folio);
 		folio_unlock(folio);
-- 
2.54.0