nilfs2: fix null-ptr-deref in block_touch_buffer tracepoint

Patch series "nilfs2: fix null-ptr-deref bugs on block tracepoints".

This series fixes null pointer dereference bugs that occur when using
nilfs2 and two block-related tracepoints.


This patch (of 2):

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.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 5305cb8 ("block: add block_{touch|dirty}_buffer tracepoint")
Signed-off-by: Ryusuke Konishi <[email protected]>
Reported-by: Ubisectech Sirius <[email protected]>
Closes: https://lkml.kernel.org/r/[email protected]
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=9982fb8d18eba905abe2
Tested-by: [email protected]
Cc: Tejun Heo <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: konis

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;
 }