[PATCH 1/3] nilfs2: fix potential null-ptr-deref in nilfs_btree_insert()

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

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/20240902084101.138971-1-lizhi.xu@windriver.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Reported-by: syzbot+9bff4c7b992038a7409f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9bff4c7b992038a7409f
Fixes: 17c76b0104e4 ("nilfs2: B-tree based block mapping")
---
 fs/nilfs2/btree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/nilfs2/btree.c b/fs/nilfs2/btree.c
index 862bdf23120e..d390b8ba00d4 100644
--- a/fs/nilfs2/btree.c
+++ b/fs/nilfs2/btree.c
@@ -381,7 +381,8 @@ static int nilfs_btree_root_broken(const struct nilfs_btree_node *node,
 	if (unlikely(level < NILFS_BTREE_LEVEL_NODE_MIN ||
 		     level >= NILFS_BTREE_LEVEL_MAX ||
 		     nchildren < 0 ||
-		     nchildren > NILFS_BTREE_ROOT_NCHILDREN_MAX)) {
+		     nchildren > NILFS_BTREE_ROOT_NCHILDREN_MAX ||
+		     (nchildren == 0 && level > NILFS_BTREE_LEVEL_NODE_MIN))) {
 		nilfs_crit(inode->i_sb,
 			   "bad btree root (ino=%lu): level = %d, flags = 0x%x, nchildren = %d",
 			   inode->i_ino, level, flags, nchildren);
-- 
2.43.0