Re: [PATCH v4 13/30] evm: add post set acl hook

From: Christian Brauner <brauner@kernel.org>
To: Mimi Zohar <zohar@linux.ibm.com>
Cc: linux-fsdevel@vger.kernel.org, Seth Forshee <sforshee@kernel.org>,
	Christoph Hellwig <hch@lst.de>, Al Viro <viro@zeniv.linux.org.uk>,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	Paul Moore <paul@paul-moore.com>
Subject: Re: [PATCH v4 13/30] evm: add post set acl hook
Date: Fri, 30 Sep 2022 10:44:38 +0200	[thread overview]
Message-ID: <20220930084438.4wuyeyogdthiwmmn@wittgenstein> (raw)
In-Reply-To: <9b71392a68d9441697fcca12b30e26578ed7423f.camel@linux.ibm.com>

On Thu, Sep 29, 2022 at 09:44:45PM -0400, Mimi Zohar wrote:
> Hi Christian,
> 
> On Thu, 2022-09-29 at 17:30 +0200, Christian Brauner wrote:
> > The security_inode_post_setxattr() hook is used by security modules to
> > update their own security.* xattrs. Consequently none of the security
> > modules operate on posix acls. So we don't need an additional security
> > hook when post setting posix acls.
> > 
> > However, the integrity subsystem wants to be informed about posix acl
> > changes and specifically evm to update their hashes when the xattrs
> > change. 
> 
> ^... to be informed about posix acl changes in order to reset the EVM
> status flag.

Substituted. 

> 
> > The callchain for evm_inode_post_setxattr() is:
> > 
> > -> evm_inode_post_setxattr()
> 
> Resets the EVM status flag for both EVM signatures and HMAC.
> 
> >    -> evm_update_evmxattr()
> 
> evm_update_evmxattr() is only called for "security.evm", not acls.  

I've added both comments but note that I'm explaining this in the
paragraph below as well.

> 
> >       -> evm_calc_hmac()
> >          -> evm_calc_hmac_or_hash()
> > 
> > and evm_cacl_hmac_or_hash() walks the global list of protected xattr
> > names evm_config_xattrnames. This global list can be modified via
> > /sys/security/integrity/evm/evm_xattrs. The write to "evm_xattrs" is
> > restricted to security.* xattrs and the default xattrs in
> > evm_config_xattrnames only contains security.* xattrs as well.
> > 
> > So the actual value for posix acls is currently completely irrelevant
> > for evm during evm_inode_post_setxattr() and frankly it should stay that
> > way in the future to not cause the vfs any more headaches. But if the
> > actual posix acl values matter then evm shouldn't operate on the binary
> > void blob and try to hack around in the uapi struct anyway. Instead it
> > should then in the future add a dedicated hook which takes a struct
> > posix_acl argument passing the posix acls in the proper vfs format.
> > 
> > For now it is sufficient to make evm_inode_post_set_acl() a wrapper
> > around evm_inode_post_setxattr() not passing any actual values down.
> > This will still cause the hashes to be updated as before.
> 
> ^This will cause the EVM status flag to be reset.

Substituted.