Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

On Fri, 2021-12-10 at 09:17 -0500, Stefan Berger wrote:

On 12/10/21 08:02, Mimi Zohar wrote:

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On Fri, 2021-12-10 at 07:40 -0500, Stefan Berger wrote:

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On 12/10/21 07:09, Mimi Zohar wrote:

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On Fri, 2021-12-10 at 12:49 +0100, Christian Brauner wrote:

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There's still the problem that if you write the policy,
making the file disappear then unmount and remount
securityfs it will come back.  My guess for fixing this is
that we only stash the policy file reference,
create it if NULL but then set the pointer to PTR_ERR(-
EINVAL) or something and refuse to create it for that
value.

Some sort of indicator that gets stashed in struct ima_ns
that the file does not get recreated on consecutive mounts.
That shouldn't be hard to fix.

The policy file disappearing is for backwards compatibility,
prior to being able to extend the custom policy.  For embedded
usecases, allowing the policy to be written exactly once might
makes sense.  Do we really want/need to continue to support
removing the policy in namespaces?

I don't have an answer but should the behavior for the same
#define in this case be different for host and namespaces? Or
should we just 'select IMA_WRITE_POLICY and IMA_READ_POLICY' when
IMA_NS is selected?

The latter option sounds good.  Being able to analyze the namespace
policy is really important.

Ok, I will adjust the Kconfig for this then. This then warrants the 
question whether to move the dentry into the ima_namespace. The
current code looks like this.

#if !defined(CONFIG_IMA_WRITE_POLICY) &&
!defined(CONFIG_IMA_READ_POLICY)
         securityfs_remove(ns->policy_dentry);
         ns->policy_dentry = NULL;
         ns->policy_dentry_removed = true;
#elif defined(CONFIG_IMA_WRITE_POLICY)

With IMA_NS selecting IMA_WRITE_POLICY and IMA_READ_POLICY the above 
wouldn't be necessary anymore but I find it 'cleaner' to still have
the dentry isolated rather than it being a global static as it was
before...

This is really, really why you don't want the semantics inside the
namespace to differ from those outside, because it creates confusion
for the people reading the code, especially with magically forced
config options like this.  I'd strongly suggest you either keep the
semantic in the namespace or eliminate it entirely.

If you really, really have to make the namespace behave differently,
then use global variables and put a big comment on that code saying it
can never be reached once CONFIG_IMA_NS is enabled.

James