Re: [PATCH v7 00/16] EVM

Mimi Zohar
2011-07-01T14:34:53Z

On Thu, 2011-06-30 at 18:31 -0400, Kyle Moffett wrote:

> The problem is that you are assuming that a large chunk of filesystem
> code is capable of properly and securely handling untrusted and
> malicious
> content. Historically filesystem drivers are NOT capable of handling
> such things, as evidenced by the large number of bugs that tools such
> as
> fsfuzzer tend to trigger. If you want to use IMA as-designed then you
> need to perform a relatively extensive audit of filesystem and fsck
> code.
>
> Furthermore, even when the filesystem does not have any security
> issues
> itself, you are assuming that intentionally malicious data-aliasing
> between "trusted" and "untrusted" files can have no potential security
> implications. You should look at the prevalence of simple stupid
> "/tmp"
> symlink attacks for more counter-examples there.
>
> In addition, IMA relies on the underlying attribute and data caching
> properties of the VFS, which won't hold true for intentionally
> malicious
> corrupted filesystems. It effectively assumes that writing data or
> metadata for one file will not invalidate the cached data or metadata
> for
> another which is blatantly false when filesystem extents overlap each
> other.
>
> Overall, the IMA architecture assumes that if it loads and validates
> the
> file data or metadata that it cannot be changed except through a
> kernel
> access to that particular inode. For a corrupted filesystem that is
> absolutely untrue.
>
> Cheers,
> Kyle Moffett

You've brought up a number of interesting scenarios, which I appreciate.
I will definitely take a closer look at fsfuzzer. It might be a good
starting point for an EVM/IMA-appraisal LTP testsuite. The bottom line,
as I said previously, is that EVM/IMA-appraisal doesn't need to prevent
these things from occurring. It just needs to be able to detect them.
Caching the integrity verification results is a performance issue, be it
an important one.

Currently the integrity verification results are reset when the file
data or metadata changes and removed on __fput(). Based on your
scenarios, I am looking to see if there might be additional situations
where the verification results need to be reset.

thanks,

Mimi