Hi Simon,

On Tue, 2021-07-27 at 16:33 +0000, THOBY Simon wrote:

IMA protects files by storing a hash (or a signature thereof) of their
content in the security.ima xattr. While the security.ima xattr itself
is protected by EVM with either a HMAC or a digital signature, no
mechanism is currently in place to ensure that the security.ima xattr
was generated with a strong digest algorithm, as was outlined in
https://lore.kernel.org/linux-integrity/10dde047d76b447f32ca91356599be679b8a76e5.camel@linux.ibm.com/t/#m0f8127c6982ef94aa42f5cc13ea83b9f9000917e (local)

Discussions should be summarized inline.  A reference to the thread
discussion may be included in a "Link" tag.  When including a "Link"
tag use the "permalink" as listed on the linux-integrity thread.  Once
the discussion is summarized, will a reference to the link really be
necessary?   Maybe fold in the subsequent paragraphs below.  Remember,
the entire cover letter or part of it, may be used as the git merge
text.

One important point is safeguarding users from mislabelling their
files when using userland utilities to update their files, as this
is the kind of behavior one can observe with evmctl (`evmctl ima_hash`
defaults to sha1). Another group that may be interested is those
that have deployed IMA years ago, possibly using algorithms that
was then deemed sufficiently collision-resistant, but that proved
to be weak with the passage of time (note that this could also
happen in the future with algorithms considered safe today).
This patch provides a migration path of sorts for these users.

This patch series gives users the ability to restrict the algorithms
accepted by their system, both when writing/updating xattrs, and
when appraising files, while retaining a permissive behavior by default
to preserve backward compatibility.

To provide these features, alter the behavior of setxattr to
only accept hashes built in the kernel, instead of any hash listed
in the kernel (complete list crypto/hash_info.c). In addition, the
user can define in his IMA policy the list of digest algorithms
allowed for writing to the security.ima xattr. In that case,
only algorithms present in that list are accepted for writing.

In addition, users may opt-in to whitelisting the hash
algorithms accepted when appraising thanks to the new
"appraise_hash" IMA policy option.
By default IMA will keep accepting any hash algorithm, but specifying
that option will make appraisal of files hashed with another algorithm
fail.


Even when using this option to restrict accepted hashes, a migration
to a new algorithm is still possible. Suppose your policy states you
must migrate from 'old_algo' (e.g. sha1) to 'new_algo' (e.g. one of
sha256/384/512). You can upgrade without relaxing the hash requirements:
alter your policy rules from 'appraise_hash=old_algo' to
'appraise_hash=old_algo,new_algo', update the "ima_hash" parameter to
'new_algo', reboot, relabel all your files with 'new_algo', alter your
policy_rule from 'appraise_hash=old_algo,new_algo' to
'appraise_hash=new_algo', reboot again and you're done.
Agreed, it's quite a lot of churn - I don't know if this can be reduced -
but this is technically doable.

Perhaps update the last line?

thanks,

Mimi