Re: [PATCH v6 03/20] evm: re-release

Serge E. Hallyn
2011-06-02T22:38:09Z

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 <serge.hallyn@canonical.com>

-serge