Re: [PATCH 23/28] security: Introduce LSM_ORDER_LAST

[Roberto Sassu]

On Wed, 2023-03-08 at 09:00 -0500, Mimi Zohar wrote:
> On Wed, 2023-03-08 at 14:26 +0100, Roberto Sassu wrote:
> > On Wed, 2023-03-08 at 08:13 -0500, Mimi Zohar wrote:
> > > Hi Roberto,
> > > 
> > > On Fri, 2023-03-03 at 19:25 +0100, Roberto Sassu wrote:
> > > > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > > 
> > > > Introduce LSM_ORDER_LAST, to satisfy the requirement of LSMs willing to be
> > > > the last, e.g. the 'integrity' LSM, without changing the kernel command
> > > > line or configuration.
> > > 
> > > Please reframe this as a bug fix for 79f7865d844c ("LSM: Introduce
> > > "lsm=" for boottime LSM selection") and upstream it first, with
> > > 'integrity' as the last LSM.   The original bug fix commit 92063f3ca73a
> > > ("integrity: double check iint_cache was initialized") could then be
> > > removed.
> > 
> > Ok, I should complete the patch by checking the cache initialization in
> > iint.c.
> > 
> > > > As for LSM_ORDER_FIRST, LSMs with LSM_ORDER_LAST are always enabled and put
> > > > at the end of the LSM list in no particular order.
> > > 
> > > ^Similar to LSM_ORDER_FIRST ...
> > > 
> > > And remove "in no particular order".
> > 
> > The reason for this is that I originally thought that the relative
> > order of LSMs specified in the kernel configuration or the command line
> > was respected (if more than one LSM specifies LSM_ORDER_LAST). In fact
> > not. To do this, we would have to parse the LSM string again, as it is
> > done for LSM_ORDER_MUTABLE LSMs.
> 
> IMA and EVM are only configurable if 'integrity' is enabled.  Similar
> to how LSM_ORDER_FIRST is reserved for capabilities, LSM_ORDER_LAST
> should be reserved for integrity (LSMs), if it is configured, for the
> reason as described in the "[PATCH 24/28] ima: Move to LSM
> infrastructure" patch description.

Yes, it is just that nothing prevents to have multiple LSMs with order
LSM_ORDER_LAST. I guess we will enforce that it is only one by
reviewing the code.

Thanks

Roberto

> > Thanks
> > 
> > Roberto
> > 
> > > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > > ---
> > > >  include/linux/lsm_hooks.h |  1 +
> > > >  security/security.c       | 12 +++++++++---
> > > >  2 files changed, 10 insertions(+), 3 deletions(-)
> > > > 
> > > > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > > > index 21a8ce23108..05c4b831d99 100644
> > > > --- a/include/linux/lsm_hooks.h
> > > > +++ b/include/linux/lsm_hooks.h
> > > > @@ -93,6 +93,7 @@ extern void security_add_hooks(struct security_hook_list *hooks, int count,
> > > >  enum lsm_order {
> > > >  	LSM_ORDER_FIRST = -1,	/* This is only for capabilities. */
> > > >  	LSM_ORDER_MUTABLE = 0,
> > > > +	LSM_ORDER_LAST = 1,
> > > >  };
> > > >  
> > > >  struct lsm_info {
> > > > diff --git a/security/security.c b/security/security.c
> > > > index 322090a50cd..24f52ba3218 100644
> > > > --- a/security/security.c
> > > > +++ b/security/security.c
> > > > @@ -284,9 +284,9 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
> > > >  		bool found = false;
> > > >  
> > > >  		for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
> > > > -			if (lsm->order == LSM_ORDER_MUTABLE &&
> > > > -			    strcmp(lsm->name, name) == 0) {
> > > > -				append_ordered_lsm(lsm, origin);
> > > > +			if (strcmp(lsm->name, name) == 0) {
> > > > +				if (lsm->order == LSM_ORDER_MUTABLE)
> > > > +					append_ordered_lsm(lsm, origin);
> > > >  				found = true;
> > > >  			}
> > > >  		}
> > > > @@ -306,6 +306,12 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
> > > >  		}
> > > >  	}
> > > >  
> > > > +	/* LSM_ORDER_LAST is always last. */
> > > > +	for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
> > > > +		if (lsm->order == LSM_ORDER_LAST)
> > > > +			append_ordered_lsm(lsm, "   last");
> > > > +	}
> > > > +
> > > >  	/* Disable all LSMs not in the ordered list. */
> > > >  	for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
> > > >  		if (exists_ordered_lsm(lsm))