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

[Roberto Sassu <roberto.sassu@huaweicloud.com>]

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;