linux-kernel - Re: [PATCH v2 00/11] ima: Fix rule parsing bugs and extend KEXEC

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Date:   Tue, 30 Jun 2020 20:29:00 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>
Cc:     James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Prakhar Srivastava <prsriva02@...il.com>,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Janne Karhunen <janne.karhunen@...il.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org, Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v2 00/11] ima: Fix rule parsing bugs and extend
 KEXEC_CMDLINE rule support

On Fri, 2020-06-26 at 17:38 -0500, Tyler Hicks wrote:
> This series ultimately extends the supported IMA rule conditionals for
> the KEXEC_CMDLINE hook function. As of today, there's an imbalance in
> IMA language conditional support for KEXEC_CMDLINE rules in comparison
> to KEXEC_KERNEL_CHECK and KEXEC_INITRAMFS_CHECK rules. The KEXEC_CMDLINE
> rules do not support *any* conditionals so you cannot have a sequence of
> rules like this:
> 
>  dont_measure func=KEXEC_KERNEL_CHECK obj_type=foo_t
>  dont_measure func=KEXEC_INITRAMFS_CHECK obj_type=foo_t
>  dont_measure func=KEXEC_CMDLINE obj_type=foo_t
>  measure func=KEXEC_KERNEL_CHECK
>  measure func=KEXEC_INITRAMFS_CHECK
>  measure func=KEXEC_CMDLINE
> 
> Instead, KEXEC_CMDLINE rules can only be measured or not measured and
> there's no additional flexibility in today's implementation of the
> KEXEC_CMDLINE hook function.
> 
> With this series, the above sequence of rules becomes valid and any
> calls to kexec_file_load() with a kernel and initramfs inode type of
> foo_t will not be measured (that includes the kernel cmdline buffer)
> while all other objects given to a kexec_file_load() syscall will be
> measured. There's obviously not an inode directly associated with the
> kernel cmdline buffer but this patch series ties the inode based
> decision making for KEXEC_CMDLINE to the kernel's inode. I think this
> will be intuitive to policy authors.
> 
> While reading IMA code and preparing to make this change, I realized
> that the buffer based hook functions (KEXEC_CMDLINE and KEY_CHECK) are
> quite special in comparison to longer standing hook functions. These
> buffer based hook functions can only support measure actions and there
> are some restrictions on the conditionals that they support. However,
> the rule parser isn't enforcing any of those restrictions and IMA policy
> authors wouldn't have any immediate way of knowing that the policy that
> they wrote is invalid. For example, the sequence of rules above parses
> successfully in today's kernel but the
> "dont_measure func=KEXEC_CMDLINE ..." rule is incorrectly handled in
> ima_match_rules(). The dont_measure rule is *always* considered to be a
> match so, surprisingly, no KEXEC_CMDLINE measurements are made.
> 
> While making the rule parser more strict, I realized that the parser
> does not correctly free all of the allocated memory associated with an
> ima_rule_entry when going down some error paths. Invalid policy loaded
> by the policy administrator could result in small memory leaks.
> 
> I envision patches 1-6 going to stable. The series is ordered in a way
> that has all the fixes up front, followed by cleanups, followed by the
> feature patch. The breakdown of patches looks like so:
> 
>  Memory leak fixes: 1-3
>  Parser strictness fixes: 4-6
>  Code cleanups made possible by the fixes: 7-10
>  Extend KEXEC_CMDLINE rule support: 11
> 
> Perhaps the most logical ordering for code review is:
> 
>  1, 2, 3, 7, 8, 4, 5, 6, 9, 10, 11
> 
> If you'd like me to re-order or split up the series, just let me know.
> Thanks for considering these patches!
> 
> * Series-wide v2 changes
>   - Rebased onto next-integrity-testing
>   - Squashed patches 2 and 3 from v1
>     + Updated this cover letter to account for changes to patch index
>       changes
>     + See patch 2 for specific code changes

Other than the comment on 9/11 the patch set looks good.

thanks!

Mimi