Re: [PATCH 1/2] ima: Pre-parse the list of keyrings in a KEY_CHECK rule

[Tyler Hicks <tyhicks@linux.microsoft.com>]

On 2020-08-06 11:34:43, Nayna wrote:
> 
> On 7/27/20 10:08 AM, Tyler Hicks wrote:
> > The ima_keyrings buffer was used as a work buffer for strsep()-based
> > parsing of the "keyrings=" option of an IMA policy rule. This parsing
> > was re-performed each time an asymmetric key was added to a kernel
> > keyring for each loaded policy rule that contained a "keyrings=" option.
> > 
> > An example rule specifying this option is:
> > 
> >   measure func=KEY_CHECK keyrings=a|b|c
> > 
> > The rule says to measure asymmetric keys added to any of the kernel
> > keyrings named "a", "b", or "c". The size of the buffer size was
> > equal to the size of the largest "keyrings=" value seen in a previously
> > loaded rule (5 + 1 for the NUL-terminator in the previous example) and
> > the buffer was pre-allocated at the time of policy load.
> > 
> > The pre-allocated buffer approach suffered from a couple bugs:
> > 
> > 1) There was no locking around the use of the buffer so concurrent key
> >     add operations, to two different keyrings, would result in the
> >     strsep() loop of ima_match_keyring() to modify the buffer at the same
> >     time. This resulted in unexpected results from ima_match_keyring()
> >     and, therefore, could cause unintended keys to be measured or keys to
> >     not be measured when IMA policy intended for them to be measured.
> > 
> > 2) If the kstrdup() that initialized entry->keyrings in ima_parse_rule()
> >     failed, the ima_keyrings buffer was freed and set to NULL even when a
> >     valid KEY_CHECK rule was previously loaded. The next KEY_CHECK event
> >     would trigger a call to strcpy() with a NULL destination pointer and
> >     crash the kernel.
> > 
> > Remove the need for a pre-allocated global buffer by parsing the list of
> > keyrings in a KEY_CHECK rule at the time of policy load. The
> > ima_rule_entry will contain an array of string pointers which point to
> > the name of each keyring specified in the rule. No string processing
> > needs to happen at the time of asymmetric key add so iterating through
> > the list and doing a string comparison is all that's required at the
> > time of policy check.
> > 
> > In the process of changing how the "keyrings=" policy option is handled,
> > a couple additional bugs were fixed:
> > 
> > 1) The rule parser accepted rules containing invalid "keyrings=" values
> >     such as "a|b||c", "a|b|", or simply "|".
> > 
> > 2) The /sys/kernel/security/ima/policy file did not display the entire
> >     "keyrings=" value if the list of keyrings was longer than what could
> >     fit in the fixed size tbuf buffer in ima_policy_show().
> > 
> > Fixes: 5c7bac9fb2c5 ("IMA: pre-allocate buffer to hold keyrings string")
> > Fixes: 2b60c0ecedf8 ("IMA: Read keyrings= option from the IMA policy")
> > Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> > ---
> >   security/integrity/ima/ima_policy.c | 138 +++++++++++++++++++---------
> >   1 file changed, 93 insertions(+), 45 deletions(-)
> > 
> > diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> > index 07f033634b27..c328cfa0fc49 100644
> > --- a/security/integrity/ima/ima_policy.c
> > +++ b/security/integrity/ima/ima_policy.c
> > @@ -59,6 +59,11 @@ enum policy_types { ORIGINAL_TCB = 1, DEFAULT_TCB };
> >   enum policy_rule_list { IMA_DEFAULT_POLICY = 1, IMA_CUSTOM_POLICY };
> > +struct ima_rule_opt_list {
> > +	size_t count;
> > +	char *items[];
> > +};
> > +
> >   struct ima_rule_entry {
> >   	struct list_head list;
> >   	int action;
> > @@ -78,7 +83,7 @@ struct ima_rule_entry {
> >   		int type;	/* audit type */
> >   	} lsm[MAX_LSM_RULES];
> >   	char *fsname;
> > -	char *keyrings; /* Measure keys added to these keyrings */
> > +	struct ima_rule_opt_list *keyrings; /* Measure keys added to these keyrings */
> >   	struct ima_template_desc *template;
> >   };
> > @@ -206,10 +211,6 @@ static LIST_HEAD(ima_policy_rules);
> >   static LIST_HEAD(ima_temp_rules);
> >   static struct list_head *ima_rules = &ima_default_rules;
> > -/* Pre-allocated buffer used for matching keyrings. */
> > -static char *ima_keyrings;
> > -static size_t ima_keyrings_len;
> > -
> >   static int ima_policy __initdata;
> >   static int __init default_measure_policy_setup(char *str)
> > @@ -253,6 +254,72 @@ static int __init default_appraise_policy_setup(char *str)
> >   }
> >   __setup("ima_appraise_tcb", default_appraise_policy_setup);
> > +static struct ima_rule_opt_list *ima_alloc_rule_opt_list(const substring_t *src)
> > +{
> > +	struct ima_rule_opt_list *opt_list;
> > +	size_t count = 0;
> > +	char *src_copy;
> > +	char *cur, *next;
> > +	size_t i;
> > +
> > +	src_copy = match_strdup(src);
> > +	if (!src_copy)
> > +		return NULL;
> 
> The caller of this function checks for IS_ERR(..) and not
> IS_ERR_OR_NULL(..). Shouldn't it return ERR_PTR(-EINVAL) instead of NULL ?

Yes! Thank you for catching this.

I switched this function to returning an ERR_PTR() towards the end of my
development for this series and missed this particular return.

I'll send out a v2 ASAP.

Tyler

> 
> Thanks & Regards,
> 
>     - Nayna