nilfs2: fix potential null-ptr-deref in nilfs_btree_insert()

nilfs2: fix potential null-ptr-deref in nilfs_btree_insert()

Patch series "nilfs2: fix potential issues with empty b-tree nodes".

This series addresses three potential issues with empty b-tree nodes that
can occur with corrupted filesystem images, including one recently
discovered by syzbot.

This patch (of 3):

If a b-tree is broken on the device, and the b-tree height is greater than
2 (the level of the root node is greater than 1) even if the number of
child nodes of the b-tree root is 0, a NULL pointer dereference occurs in
nilfs_btree_prepare_insert(), which is called from nilfs_btree_insert().

This is because, when the number of child nodes of the b-tree root is 0,
nilfs_btree_do_lookup() does not set the block buffer head in any of
path[x].bp_bh, leaving it as the initial value of NULL, but if the level
of the b-tree root node is greater than 1, nilfs_btree_get_nonroot_node(),
which accesses the buffer memory of path[x].bp_bh, is called.

Fix this issue by adding a check to nilfs_btree_root_broken(), which
performs sanity checks when reading the root node from the device, to
detect this inconsistency.

Thanks to Lizhi Xu for trying to solve the bug and clarifying the cause
early on.

Link: https://lkml.kernel.org/r/20240904081401.16682-1-konishi.ryusuke@gmail.com
Link: https://lkml.kernel.org/r/20240902084101.138971-1-lizhi.xu@windriver.com
Link: https://lkml.kernel.org/r/20240904081401.16682-2-konishi.ryusuke@gmail.com
Fixes: 17c76b0104e4 ("nilfs2: B-tree based block mapping")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+9bff4c7b992038a7409f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9bff4c7b992038a7409f
Cc: Lizhi Xu <lizhi.xu@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>