linux-kernel - Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY

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Message-ID: <c1185901c99091a29a865f7a862bc979873301ad.camel@huaweicloud.com>
Date: Tue, 25 Mar 2025 18:01:09 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Frederick Lawler <fred@...udflare.com>
Cc: Mimi Zohar <zohar@...ux.ibm.com>, Dmitry Kasatkin
 <dmitry.kasatkin@...il.com>, Roberto Sassu <roberto.sassu@...wei.com>, Eric
 Snowberg <eric.snowberg@...cle.com>, James Morris
 <james.l.morris@...cle.com>, "Serge E. Hallyn" <serge@...lyn.com>,
 linux-ima-devel@...ts.sourceforge.net, 
 linux-ima-user@...ts.sourceforge.net,
 linux-security-module@...r.kernel.org,  linux-kernel@...r.kernel.org,
 linux-team@...udflare.com
Subject: Re: [PATCH] ima: process_measurement() needlessly takes
 inode_lock() on MAY_READ

On Tue, 2025-03-25 at 11:42 -0500, Frederick Lawler wrote:
> On Tue, Mar 25, 2025 at 05:30:32PM +0100, Roberto Sassu wrote:
> > On 3/25/2025 4:58 PM, Frederick Lawler wrote:
> > > On IMA policy update, if a measure rule exists in the policy,
> > > IMA_MEASURE is set for ima_policy_flags which makes the violation_check
> > > variable always true. Coupled with a no-action on MAY_READ for a
> > > FILE_CHECK call, we're always taking the inode_lock().
> > > 
> > > This becomes a performance problem for extremely heavy read-only workloads.
> > > Therefore, prevent this only in the case there's no action to be taken.
> > > 
> > > Signed-off-by: Frederick Lawler <fred@...udflare.com>
> > > ---
> > >   security/integrity/ima/ima_main.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > > index 2aebb7984437..78921e69ee14 100644
> > > --- a/security/integrity/ima/ima_main.c
> > > +++ b/security/integrity/ima/ima_main.c
> > > @@ -181,7 +181,7 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
> > >   	action = ima_get_action(inode, mask, func, &pcr);
> > >   	violation_check = ((func == FILE_CHECK || func == MMAP_CHECK) &&
> > >   			   (ima_policy_flag & IMA_MEASURE));
> > > -	if (!action && !violation_check)
> > > +	if (!action && (mask == MAY_READ || !violation_check))
> > >   		return 0;
> > 
> 
> Hi Roberto,
> 
> > Hi Frederick
> > 
> > thanks, nice catch!
> > 
> > Thinking... in fact you are saying that there are conditions for which
> > ima_rdwr_violation_check() does nothing.
> > 
> > For better clarity, I would add the conditions for which we are doing a
> > violation check in violation_check directly. So that, one can just go to the
> > function and see that in fact nothing special is done other than doing the
> > same checks in advance before taking the lock (the conditions you are
> > checking on are immutable, so it is fine).
> > 
> > So, it is not a write, and the file is not being measured (this would be a
> > bit redundant given that we are checking anyway !action).
> > 
> > Thanks
> > 
> 
> The ima_rdwr_violation_check() call takes a action & IMA_MEASURE
> argument anyway.
> 
> My initial thought was to replace ima_flag_policy & IMA_MEASURE with
> action & IMA_MEASURE there, but I wasn't sure if there was a race
> problem that the ima_rdwr_violation_check() is trying to catch for the non
> FILE_CHECK cases.

Let's keep as it is for now.

> Otherwise, I think the checks in the ima_rdwr_violation_check() demand the lock,
> and therefore we can't just move them out to that violation_check
> variable--unless I'm missing something. As for other conditions, I think
> it's _just_ the MAY_READ we care about.

Yes, of course.

I meant, since in ima_rdwr_violation_check() there is:


if (mode & FMODE_WRITE)
...
else if (... && must_measure)


which don't need to be under lock, then I would have modified
violation_check to:

	violation_check = ((func == FILE_CHECK || func == MMAP_CHECK ||
			    func == MMAP_CHECK_REQPROT) &&
			   (ima_policy_flag & IMA_MEASURE)) &&
			   ((action & IMA_MEASURE) || (mask & MAY_WRITE));

Roberto

> Is what you're suggesting to move the check mask == MAY_READ to instead be in
> that violation_check variable than the branch?
> 
> > Roberto
> > 
> > >   	must_appraise = action & IMA_APPRAISE;
> > 
> 
> Thanks,
> Fred