fscrypt: Add missing superblock check in find_or_insert_direct_key()

Overview

In the Linux kernel, the following vulnerability has been resolved:

fscrypt: Add missing superblock check in find_or_insert_direct_key()

The legacy 'fscrypt_direct_keys' table caches master keys that are used by v1 encryption policies that have FSCRYPT_POLICY_FLAG_DIRECT_KEY. It's just a global table for all filesystems (since the keys can be provided by the legacy process-subscribed keyrings mechanism, which makes it difficult to reuse super_block::s_master_keys).

The entries in it ('struct fscrypt_direct_key') do contain a super_block pointer, though, for passing to fscrypt_destroy_inline_crypt_key() when the last inode that references the key is evicted.

However, when finding the fscrypt_direct_key for an inode, we weren't actually comparing the super_block pointer. As a result, inodes with different super_blocks could point to the same fscrypt_direct_key. That could extend the lifetime of a fscrypt_direct_key beyond the super_block it points to, causing a use-after-free later.

Fix this by creating distinct fscrypt_direct_key structs for distinct super_block structs.

Note that this problem doesn't exist in the v2 policy equivalent ("per-mode keys"), since the data structures there are per super_block.

Affected

• Linux >= 22e9947a4b2ba255888541bd0111cf00b9b16586 < 965b5bc8cf5031225e057979ce660fec2bd5fbfc
• Linux >= 22e9947a4b2ba255888541bd0111cf00b9b16586 < 330249609b70778094a7a36f5b6bcfa6362121d4
• Linux >= 22e9947a4b2ba255888541bd0111cf00b9b16586 < deff41898a5ae3a47db5fa1896a494aa95efda5d
• Linux >= 22e9947a4b2ba255888541bd0111cf00b9b16586 < 95376fe9c145be35566991df99c53134943d992f
• Linux >= 22e9947a4b2ba255888541bd0111cf00b9b16586 < 466f187b501a5ac8e1ea2ccf3ccd5c46108d8830
• Linux >= 22e9947a4b2ba255888541bd0111cf00b9b16586 < b5fa40226e71c17847b9ff2816c6ca4133d0d994
• Linux 6.1

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.