Re: [PATCH v2 08/12] evm: Allow xattr/attr operations for portable signatures if check fails

From: Mimi Zohar
Date: Thu Sep 17 2020 - 09:04:50 EST


Hi Roberto,

"if check fails" in the Subject line is unnecessary.

On Fri, 2020-09-04 at 11:26 +0200, Roberto Sassu wrote:
> If files with portable signatures are copied from one location to another
> or are extracted from an archive, verification can temporarily fail until
> all xattrs/attrs are set in the destination. Only portable signatures may
> be moved or copied from one file to another, as they don't depend on
> system-specific information such as the inode generation. Instead portable
> signatures must include security.ima.
>
> Unlike other security.evm types, EVM portable signatures are also
> immutable. Thus, it wouldn't be a problem to allow xattr/attr operations
> when verification fails, as portable signatures will never be replaced with
> an HMAC on possibly corrupted xattrs/attrs.
>
> This patch first introduces a new integrity status called
> INTEGRITY_FAIL_IMMUTABLE, that allows callers of
> evm_verify_current_integrity() to detect that a portable signature didn't
> pass verification and then adds an exception in evm_protect_xattr() and
> evm_inode_setattr() for this status and returns 0 instead of -EPERM.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>

< snip >

> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 05be1ad3e6f3..a5dab1ac9374 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
>
> @@ -358,6 +364,12 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
> -EPERM, 0);
> }
> out:
> + /* Writing other xattrs is safe for portable signatures, as portable
> + * signatures are immutable and can never be updated.
> + */

This is the second time I'm seeing this comment format style. Why?
What changed?

Mimi

> + if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> + return 0;
> +
> if (evm_status != INTEGRITY_PASS)
> integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
> dentry->d_name.name, "appraise_metadata",