[PATCH] Squashfs: check the inode number is not the invalid value of zero

[Phillip Lougher]

Syskiller has produced an out of bounds access in fill_meta_index().

That out of bounds access is ultimately caused because the inode
has an inode number with the invalid value of zero, which was not checked.

The reason this causes the out of bounds access is due to following
sequence of events:

1. Fill_meta_index() is called to allocate (via empty_meta_index())
   and fill a metadata index.  It however suffers a data read error
   and aborts, invalidating the newly returned empty metadata index.
   It does this by setting the inode number of the index to zero,
   which means unused (zero is not a valid inode number).

2. When fill_meta_index() is subsequently called again on another
   read operation, locate_meta_index() returns the previous index
   because it matches the inode number of 0.  Because this index
   has been returned it is expected to have been filled, and because
   it hasn't been, an out of bounds access is performed.

This patch adds a sanity check which checks that the inode number
is not zero when the inode is created and returns -EINVAL if it is.

Reported-by: "Ubisectech Sirius" <bugreport@xxxxxxxxxxxxxx>
Signed-off-by: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/87f5c007-b8a5-41ae-8b57-431e924c5915.bugreport@xxxxxxxxxxxxxx/
---
 fs/squashfs/inode.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/inode.c b/fs/squashfs/inode.c
index aa3411354e66..e0896d8a3cc6 100644
--- a/fs/squashfs/inode.c
+++ b/fs/squashfs/inode.c
@@ -48,6 +48,10 @@ static int squashfs_new_inode(struct super_block *sb, struct inode *inode,
 	gid_t i_gid;
 	int err;
 
+	inode->i_ino = le32_to_cpu(sqsh_ino->inode_number);
+	if(inode->i_ino == 0)
+		return -EINVAL;
+
 	err = squashfs_get_id(sb, le16_to_cpu(sqsh_ino->uid), &i_uid);
 	if (err)
 		return err;
@@ -58,7 +62,6 @@ static int squashfs_new_inode(struct super_block *sb, struct inode *inode,
 
 	i_uid_write(inode, i_uid);
 	i_gid_write(inode, i_gid);
-	inode->i_ino = le32_to_cpu(sqsh_ino->inode_number);
 	inode_set_mtime(inode, le32_to_cpu(sqsh_ino->mtime), 0);
 	inode_set_atime(inode, inode_get_mtime_sec(inode), 0);
 	inode_set_ctime(inode, inode_get_mtime_sec(inode), 0);
-- 
2.39.2