linux-kernel - Re: [RFC PATCH] selinux: add a fallback to defcontext for native labeling

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Date:   Tue, 02 Oct 2018 11:48:19 -0700
From:   Taras Kondratiuk <takondra@...co.com>
To:     Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Cc:     linux-kernel@...r.kernel.org, selinux@...ho.nsa.gov,
        xe-linux-external@...co.com
Subject: Re: [RFC PATCH] selinux: add a fallback to defcontext for native labeling

Quoting Stephen Smalley (2018-09-21 07:40:58)
> If we set the inode sid to the superblock def_sid on an invalid
> context, then we lose the association to the original context value.
> The support for deferred mapping of contexts requires allocating a new
> SID for the invalid context and storing that SID in the inode, so that
> if the context later becomes valid upon a policy update/reload, the
> inode SID will refer to the now valid context.
> To combine the two, we would need security_context_to_sid_core() to
> save the def_sid in the context structure for invalid contexts, and
> change sidtab_search_core() to use that value instead of
> SECINITSID_UNLABELED for invalid SIDs.  Then the inode would be
> treated as having the defcontext for access control and getxattr() w/o
> CAP_MAC_ADMIN purposes, but a subsequent policy update/reload that
> makes the context valid would automatically cause subsequent accesses
> to the inode to start using the original context value for access
> control and getxattr() purposes.  I think that's the behavior you
> want.
> 

While implementing the change I've realized that storing default context
for sidtab_search_core() in the context structure is not enough to
achieve the desired behavior. The same invalid context may exist in two
mounts with different 'defcontext', so default context can't be a
property of a context structure.

One way to address it is to propagate default context to sidtab_search() all
the way from inode hooks. But that will be a bit intrusive. Something like
avc_has_perm_default() will need to be added.

Other way is to check for context validity in inode hooks and provide a default
context to avc_has_perm() if the inode's sid is invalid. But this may have
performance implication since validity check will be done each time in fast
path.

Do you see any other options?