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

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

When using the "block:block_dirty_buffer" tracepoint,
mark_buffer_dirty() may cause a NULL pointer dereference, or a general
protection fault when KASAN is enabled.

This happens because, since the tracepoint was added in
mark_buffer_dirty(), 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, nilfs_grab_buffer(), which grabs a buffer
to read (or create) a block of metadata, including b-tree node blocks,
does not set the block device, but instead does so only if the buffer is
not in the "uptodate" state for each of its caller block reading
functions.  However, if the uptodate flag is set on a folio/page, and
the buffer heads are detached from it by try_to_free_buffers(), and new
buffer heads are then attached by create_empty_buffers(), the uptodate
flag may be restored to each buffer without the block device being set
to bh->b_bdev, and mark_buffer_dirty() may be called later in that
state, resulting in the bug mentioned above.

Fix this issue by making nilfs_grab_buffer() always set the block device
of the super block structure to the buffer head, regardless of the state
of the buffer's uptodate flag.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Fixes: 5305cb830834 ("block: add block_{touch|dirty}_buffer tracepoint")
Cc: stable@vger.kernel.org
---
 fs/nilfs2/btnode.c  | 2 --
 fs/nilfs2/gcinode.c | 4 +---
 fs/nilfs2/mdt.c     | 1 -
 fs/nilfs2/page.c    | 1 +
 4 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/fs/nilfs2/btnode.c b/fs/nilfs2/btnode.c
index 57b4af5ad646..501ad7be5174 100644
--- a/fs/nilfs2/btnode.c
+++ b/fs/nilfs2/btnode.c
@@ -68,7 +68,6 @@ nilfs_btnode_create_block(struct address_space *btnc, __u64 blocknr)
 		goto failed;
 	}
 	memset(bh->b_data, 0, i_blocksize(inode));
-	bh->b_bdev = inode->i_sb->s_bdev;
 	bh->b_blocknr = blocknr;
 	set_buffer_mapped(bh);
 	set_buffer_uptodate(bh);
@@ -133,7 +132,6 @@ int nilfs_btnode_submit_block(struct address_space *btnc, __u64 blocknr,
 		goto found;
 	}
 	set_buffer_mapped(bh);
-	bh->b_bdev = inode->i_sb->s_bdev;
 	bh->b_blocknr = pblocknr; /* set block address for read */
 	bh->b_end_io = end_buffer_read_sync;
 	get_bh(bh);
diff --git a/fs/nilfs2/gcinode.c b/fs/nilfs2/gcinode.c
index 1c9ae36a03ab..ace22253fed0 100644
--- a/fs/nilfs2/gcinode.c
+++ b/fs/nilfs2/gcinode.c
@@ -83,10 +83,8 @@ int nilfs_gccache_submit_read_data(struct inode *inode, sector_t blkoff,
 		goto out;
 	}
 
-	if (!buffer_mapped(bh)) {
-		bh->b_bdev = inode->i_sb->s_bdev;
+	if (!buffer_mapped(bh))
 		set_buffer_mapped(bh);
-	}
 	bh->b_blocknr = pbn;
 	bh->b_end_io = end_buffer_read_sync;
 	get_bh(bh);
diff --git a/fs/nilfs2/mdt.c b/fs/nilfs2/mdt.c
index 432181cfb0b5..965b5ad1c0df 100644
--- a/fs/nilfs2/mdt.c
+++ b/fs/nilfs2/mdt.c
@@ -92,7 +92,6 @@ static int nilfs_mdt_create_block(struct inode *inode, unsigned long block,
 	if (buffer_uptodate(bh))
 		goto failed_bh;
 
-	bh->b_bdev = sb->s_bdev;
 	err = nilfs_mdt_insert_new_block(inode, block, bh, init_block);
 	if (likely(!err)) {
 		get_bh(bh);
diff --git a/fs/nilfs2/page.c b/fs/nilfs2/page.c
index bdc4b152f6e9..bd86447d2edf 100644
--- a/fs/nilfs2/page.c
+++ b/fs/nilfs2/page.c
@@ -63,6 +63,7 @@ struct buffer_head *nilfs_grab_buffer(struct inode *inode,
 		folio_put(folio);
 		return NULL;
 	}
+	bh->b_bdev = inode->i_sb->s_bdev;
 	return bh;
 }
 
-- 
2.43.0