Quoting Mimi Zohar (zohar@linux.vnet.ibm.com):

EVM protects a file's security extended attributes(xattrs) against integrity
attacks.  This patchset provides the framework and an initial method.  The
initial method maintains an HMAC-sha1 value across the security extended
attributes, storing the HMAC value as the extended attribute 'security.evm'.
Other methods of validating the integrity of a file's metadata will be posted
separately (eg. EVM-digital-signatures).

While this patchset does authenticate the security xattrs, and
cryptographically binds them to the inode, coming extensions will bind other
directory and inode metadata for more complete protection.  To help simplify
the review and upstreaming process, each extension will be posted separately
(eg. IMA-appraisal, IMA-appraisal-directory).  For a general overview of the
proposed Linux integrity subsystem, refer to Dave Safford's whitepaper:
http://downloads.sf.net/project/linux-ima/linux-ima/Integrity_overview.pdf.

EVM depends on the Kernel Key Retention System to provide it with a
trusted/encrypted key for the HMAC-sha1 operation. The key is loaded onto the
root's keyring using keyctl.  Until EVM receives notification that the key has
been successfully loaded onto the keyring (echo 1 > <securityfs>/evm), EVM can
not create or validate the 'security.evm' xattr, but returns INTEGRITY_UNKNOWN.
Loading the key and signaling EVM should be done as early as possible. Normally
this is done in the initramfs, which has already been measured as part of the
trusted boot.  For more information on creating and loading existing
trusted/encrypted keys, refer to Documentation/keys-trusted-encrypted.txt.

Based on the LSMs enabled, the set of EVM protected security xattrs is defined
at compile.  EVM adds the following three calls to the existing security hooks:
evm_inode_setxattr(), evm_inode_post_setxattr(), and evm_inode_removexattr.  To
initialize and update the 'security.evm' extended attribute, EVM defines three
calls: evm_inode_post_init(), evm_inode_post_setattr() and
evm_inode_post_removexattr() hooks.  To verify the integrity of a security
xattr, EVM exports evm_verifyxattr().

Changelog v6: (based on Serge Hallyn's review)
- fix URL in patch description

It's still wrong in Documentation/ABI/testing/evm...

- remove evm_hmac_size definition
- use SHA1_DIGEST_SIZE (removed both MAX_DIGEST_SIZE and evm_hmac_size)
- moved linux include before other includes
- test for crypto_hash_setkey failure
- fail earlier for invalid key
- clear entire encrypted key, even on failure
- check xattr name length before comparing xattr names

The rest looks good, thanks

Acked-by: Serge Hallyn <redacted>

-serge