[PATCH 1/2] nilfs2: fix null-ptr-deref in block_touch_buffer tracepoint

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

It has been reported that when using "block:block_touch_buffer"
tracepoint, touch_buffer() called from __nilfs_get_folio_block() causes
a NULL pointer dereference, or a general protection fault when KASAN is
enabled.

This happens because since the tracepoint was added in touch_buffer(),
it references the dev_t member bh->b_bdev->bd_dev regardless of whether
the buffer head has a pointer to a block_device structure.  In the
current implementation, the block_device structure is set after the
function returns to the caller.

Here, touch_buffer() is used to mark the folio/page that owns the buffer
head as accessed, but the common search helper for folio/page used by
the caller function was optimized to mark the folio/page as accessed
when it was reimplemented a long time ago, eliminating the need to call
touch_buffer() here in the first place.

So this solves the issue by eliminating the touch_buffer() call itself.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: Ubisectech Sirius <bugreport@valiantsec.com>
Closes: https://lkml.kernel.org/r/86bd3013-887e-4e38-960f-ca45c657f032.bugreport@valiantsec.com
Reported-by: syzbot+9982fb8d18eba905abe2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9982fb8d18eba905abe2
Fixes: 5305cb830834 ("block: add block_{touch|dirty}_buffer tracepoint")
Tested-by: syzbot+9982fb8d18eba905abe2@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
---
 fs/nilfs2/page.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/nilfs2/page.c b/fs/nilfs2/page.c
index e48079ebe939..bdc4b152f6e9 100644
--- a/fs/nilfs2/page.c
+++ b/fs/nilfs2/page.c
@@ -39,7 +39,6 @@ static struct buffer_head *__nilfs_get_folio_block(struct folio *folio,
 	first_block = (unsigned long)index << (PAGE_SHIFT - blkbits);
 	bh = get_nth_bh(bh, block - first_block);
 
-	touch_buffer(bh);
 	wait_on_buffer(bh);
 	return bh;
 }
-- 
2.43.0