[PATCH v1 1/2] ima: fail signature verification on untrusted filesystems

On Mon, 19 Feb 2018, Eric W. Biederman wrote:

Mimi Zohar [off-list ref] writes:

quoted

Files on untrusted filesystems, such as fuse, can change at any time,
making the measurement(s) and by extension signature verification
meaningless.

Filesystems with servers?
Remote filesystems?
Perhaps unexpected changes.

Untrusted sounds a bit harsh, and I am not certain it quite captures
what you are looking to avoid.

Right -- I think whether you trust a filesystem or not depends on how much 
assurance you have in your specific configuration, rather than whether you 
think the filesystem can be manipulated or not.

There is a difference between:

  - This fs has no way to communicate a change to IMA, and;

  - This fs could be malicious.

In the latter case, I suggest that any fs could be malicious if the 
overall security policy / settings are inadequate for the threat model, or 
if there are vulnerabilities which allow such security to be bypassed.

Whether a user trusts FUSE on their particular system should be a policy 
decision on the part of the user.  The kernel should not be deciding what 
is trusted or not trusted here.



-- 
James Morris
[off-list ref]

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