File system metadata protection
Techniques are disclosed relating to securely storing file system metadata in a computing device. In one embodiment, a computing device includes a processor, memory, and a secure circuit. The memory has a file system stored therein that includes metadata for accessing a plurality of files in the memory. The metadata is encrypted with a metadata encryption key that is stored in an encrypted form. The secure circuit is configured to receive a request from the processor to access the file system. In response to the request, the secure circuit is configured to decrypt the encrypted form of the metadata encryption key. In some embodiments, the computing device includes a memory controller configured to receive the metadata encryption key from the secure circuit, retrieve the encrypted metadata from the memory, and decrypt the encrypted metadata prior to providing the metadata to the processor.
1. A computing device, comprising:
a processor;
memory having a first file system stored therein, wherein the first file system includes metadata for accessing a plurality of files in the memory, wherein the metadata includes directory records of the first file system, and wherein the metadata is encrypted with a metadata encryption key that is stored in an encrypted form; and
a secure circuit having cryptographic circuitry isolated from direct access by the processor, wherein the secure circuit is configured to:
receive a request from the processor to access the first file system; and
in response to the request, decrypt, via the cryptographic circuitry, the encrypted form of the metadata encryption key; and
a memory controller circuit coupled to the memory, wherein the memory controller circuit is configured to:
receive the metadata encryption key from the secure circuit;
retrieve the encrypted metadata from the memory; and
decrypt the encrypted metadata prior to providing the decrypted metadata to the processor.
2. The computing device of claim 1 , wherein the secure circuit is further configured to:
prior to providing the metadata encryption key to the memory controller circuit, encrypt the metadata encryption key with a shared encryption key known to the memory controller circuit.
3. The computing device of claim 1 , wherein the memory controller circuit is further configured to:
receive a request from the processor to write new metadata of the first file system to the memory;
encrypt the new metadata with the metadata encryption key received from the secure circuit; and
send the encrypted new metadata to the memory for storage.
4. The computing device of claim 3 , wherein the new metadata includes a record for a directory, wherein the record identifies names for a plurality of files stored in the directory.
5. The computing device of claim 1 , wherein the secure circuit is further configured to:
receive a credential supplied by a user of the computing device; and
based on the credential, derive a decryption key for decrypting the encrypted form of the metadata encryption key.
6. The computing device of claim 5 , further comprising:
a touch screen configured to:
receive a passcode from the user; and
provide the passcode to the secure circuit as the credential.
7. The computing device of claim 5 , wherein the secure circuit is further configured to:
store a unique identifier indicative of the computing device; and
derive the decryption key based on the supplied credential and the stored unique identifier.
8. The computing device of claim 1 , wherein the memory has a first partition and a second partition stored therein, wherein the first partition includes encrypted metadata of a first system that is encrypted with a first metadata encryption key, and wherein the second partition includes encrypted metadata of a second file system that is encrypted with a second metadata encryption key.
9. The computing device of claim 1 , wherein the secure circuit is further configured to communicate with the processor via a mailbox mechanism configured to isolate circuitry of the secure circuit from being accessed by the processor.