Re: [PATCH v8 22/24] evm: Make it independent from 'integrity' LSM

[Mimi Zohar <zohar@linux.ibm.com>]

On Thu, 2023-12-14 at 18:08 +0100, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
> 
> Define a new structure for EVM-specific metadata, called evm_iint_cache,
> and embed it in the inode security blob. Introduce evm_iint_inode() to
> retrieve metadata, and register evm_inode_alloc_security() for the
> inode_alloc_security LSM hook, to initialize the structure (before
> splitting metadata, this task was done by iint_init_always()).
> 
> Keep the non-NULL checks after calling evm_iint_inode() except in
> evm_inode_alloc_security(), to take into account inodes for which
> security_inode_alloc() was not called. When using shared metadata,
> obtaining a NULL pointer from integrity_iint_find() meant that the file
> wasn't processed by IMA.

^wasn't in policy.

Ok.  So now regardless of the IMA policy, EVM always allocates and
stores the EVM status.  Depending on the IMA policy, the EVM status
could be saved for a lot more inodes.

> 
> Given that from now on EVM relies on its own metadata, remove the iint
> parameter from evm_verifyxattr(). Also, directly retrieve the iint in
> evm_verify_hmac(), called by both evm_verifyxattr() and
> evm_verify_current_integrity(), since now there is no performance penalty
> in retrieving EVM metadata (constant time).

Ok.  So the change only negatively impacts memory usage, not
performance.

> 
> Replicate the management of the IMA_NEW_FILE flag (now EVM_NEW_FILE), by
> introducing evm_post_path_mknod() and evm_file_free() to respectively set
> and clear the new flag at the same time IMA does.

nit:  Instead of "(now EVM_NEW_FILE)", add an additional sentence,
saying "Define EVM_NEW_FILE".

> A noteworthy difference
> is that evm_post_path_mknod() cannot check if a file must be appraised.

This is the result of making EVM independent of IMA's policy.  
Somewhere, here or above, this needs to be stated.

> Also, since IMA_NEW_FILE is always cleared in ima_check_last_writer() if it
> is set, it is not necessary to maintain an inode version in EVM to
> replicate the IMA logic (the inode version check is in OR).

IMA checking the i_version is to prevent unnecessarily having to re-
calculate the file data hash, which depending on the file size could
take a while.   This is unnecessary for EVM, as re-calculating the EVM
hmac is triggered anytime a trusted xattr is updated.  So only the EVM
new file flag needs to cleared on file free.

> Also, move the EVM-specific flag EVM_IMMUTABLE_DIGSIG to
> security/integrity/evm/evm.h, since that definition is now unnecessary in
> the common integrity layer.
> 
> Finally, switch to the LSM reservation mechanism for the EVM xattr, and
> consequently decrement by one the number of xattrs to allocate in
> security_inode_init_security().
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>

-- 
thanks,

Mimi