CVE-2023-53536

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

blk-crypto: make blk_crypto_evict_key() more robust

If blk_crypto_evict_key() sees that the key is still in-use (due to a

bug) or that ->keyslot_evict failed, it currently just returns while

leaving the key linked into the keyslot management structures.

However, blk_crypto_evict_key() is only called in contexts such as inode

eviction where failure is not an option. So actually the caller

proceeds with freeing the blk_crypto_key regardless of the return value

of blk_crypto_evict_key().

These two assumptions don't match, and the result is that there can be a

use-after-free in blk_crypto_reprogram_all_keys() after one of these

errors occurs. (Note, these errors *shouldn't* happen; we're just

talking about what happens if they do anyway.)

Fix this by making blk_crypto_evict_key() unlink the key from the

keyslot management structures even on failure.

Also improve some comments.