Re: [RESEND][PATCH] Documentation: added order requirement for ima_hash=

[Guozihua (Scott)]

On 2022/1/26 9:07, Mimi Zohar wrote:
> On Tue, 2022-01-25 at 17:02 +0800, GUO Zihua wrote:
> > From: Guo Zihua <guozihua@xxxxxxxxxx>
> >
> > Commandline parameter ima_hash= and ima_template= has order requirement
> > for them to work correctly together. Namely ima_hash= must be
> > specified after ima_template=, otherwise ima_template= will be ignored.
> >
> > The reason is that when handling ima_hash=, ima template would be set to
> > the default value if it has not been initialized already, and that value
> > cannot be changed afterwards by ima_template=.
> >
> > This patch adds this limitation to the documentation.
> >
> > Reviewed-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > Signed-off-by: Guo Zihua <guozihua@xxxxxxxxxx>
>
> This issue should be limited to the original "ima" template format,
> which only supports hash algorithms of 20 bytes or less.  The "ima_ng"
> template has been the default since larger digests and templates were
> upstreamed back in Linux 3.13[1]. Do you really still have kernels
> built with the original "ima" template?
>
> [1] Refer to commit 4286587dccd4 ("ima: add Kconfig default measurement
> list template").
>
> thanks,
>
> Mimi

Hi Mimi,

The issue is that if ima_hash is specified before ima_template, 
ima_template will not work. Built-in default only affects which template 
will be loaded eventually.

For example, if the built-in default template is ima-ng and user would 
like to change it to ima-sig with sha512 by specifying "ima_hash=sha512 
ima_template=ima-sig" in command line, the result will be ima-ng with 
sha512, not ima-sig with sha512.

--
Best
GUO Zihua

> > ---
> >   Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index f5a27f067db9..1b5aa6ca65f8 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1843,6 +1843,10 @@
> >   			The list of supported hash algorithms is defined
> >   			in crypto/hash_info.h.
> >   
> > +			This parameter must be specified after ima_template=,
> > +			as it would set the default template and that cannot be
> > +			changed by ima_template= afterwards.
> > +
> >   	ima_policy=	[IMA]
> >   			The builtin policies to load during IMA setup.
> >   			Format: "tcb | appraise_tcb | secure_boot |
> > @@ -1879,6 +1883,9 @@
> >   			Formats: { "ima" | "ima-ng" | "ima-sig" }
> >   			Default: "ima-ng"
> >   
> > +			This parameter must be specified before ima_hash=.
> > +			Please refer to ima_hash= for further explanation.
> > +
> >   	ima_template_fmt=
> >   			[IMA] Define a custom template format.
> >   			Format: { "field1|...|fieldN" }
>
>
> .