[PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Frederick Lawler]

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@cloudflare.com>
---
Changes since v1:
- Add MAY_WRITE && action check to violation_check to avoid MAY_READ
  only situations
---
 security/integrity/ima/ima_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 9b87556b03a7..11a91d9dae48 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -237,7 +237,8 @@ static int process_measurement(struct file *file, const struct cred *cred,
 				&allowed_algos);
 	violation_check = ((func == FILE_CHECK || func == MMAP_CHECK ||
 			    func == MMAP_CHECK_REQPROT) &&
-			   (ima_policy_flag & IMA_MEASURE));
+			   (ima_policy_flag & IMA_MEASURE) &&
+			   ((action & IMA_MEASURE) || (mask & MAY_WRITE)));
 	if (!action && !violation_check)
 		return 0;
 
-- 
2.43.0

Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Frederick Lawler]

My mistake, this is PATCH v2. I forgot to change the subject in git
send-email. I can resend if that's needed.

Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Roberto Sassu]

On 3/25/2025 7:18 PM, Frederick Lawler wrote:
> My mistake, this is PATCH v2. I forgot to change the subject in git
> send-email. I can resend if that's needed.

No problem. However, let's change:

mask & MAY_WRITE

with

file->f_mode & FMODE_WRITE

Thanks

Roberto

[PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Frederick Lawler]

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@cloudflare.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;
 
 	must_appraise = action & IMA_APPRAISE;
-- 
2.43.0

Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Roberto Sassu]

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@cloudflare.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 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

Roberto

>   	must_appraise = action & IMA_APPRAISE;

Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Frederick Lawler]

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@cloudflare.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.

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.

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

Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Roberto Sassu]

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@cloudflare.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

Re: [PATCH] ima: process_measurement() needlessly takes inode_lock() on MAY_READ

[Frederick Lawler]

On Tue, Mar 25, 2025 at 06:01:09PM +0100, Roberto Sassu wrote:
> 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@cloudflare.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));
>

Sounds good! I'll make the change and submit a v2.

> 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
>