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LSM: SafeSetID: verify transitive constrainedness

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Someone might write a ruleset like the following, expecting that it
securely constrains UID 1 to UIDs 1, 2 and 3:

    1:2
    1:3

However, because no constraints are applied to UIDs 2 and 3, an attacker
with UID 1 can simply first switch to UID 2, then switch to any UID from
there. The secure way to write this ruleset would be:

    1:2
    1:3
    2:2
    3:3

, which uses "transition to self" as a way to inhibit the default-allow
policy without allowing anything specific.

This is somewhat unintuitive. To make sure that policy authors don't
accidentally write insecure policies because of this, let the kernel verify
that a new ruleset does not contain any entries that are constrained, but
transitively unconstrained.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Micah Morton <mortonm@chromium.org>
This commit is contained in:
Jann Horn 2019-04-11 13:12:43 -07:00 committed by Micah Morton
parent fbd9acb2dc
commit 4f72123da5
2 changed files with 40 additions and 2 deletions
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38
security/safesetid/securityfs.c
View File

@ -76,6 +76,37 @@ static void release_ruleset(struct setuid_ruleset *pol)
call_rcu(&pol->rcu, __release_ruleset);
}
static void insert_rule(struct setuid_ruleset *pol, struct setuid_rule *rule)
{
hash_add(pol->rules, &rule->next, __kuid_val(rule->src_uid));
}
static int verify_ruleset(struct setuid_ruleset *pol)
{
int bucket;
struct setuid_rule *rule, *nrule;
int res = 0;
hash_for_each(pol->rules, bucket, rule, next) {
if (_setuid_policy_lookup(pol, rule->dst_uid, INVALID_UID) ==
SIDPOL_DEFAULT) {
pr_warn("insecure policy detected: uid %d is constrained but transitively unconstrained through uid %d\n",
__kuid_val(rule->src_uid),
__kuid_val(rule->dst_uid));
res = -EINVAL;
/* fix it up */
nrule = kmalloc(sizeof(struct setuid_rule), GFP_KERNEL);
if (!nrule)
return -ENOMEM;
nrule->src_uid = rule->dst_uid;
nrule->dst_uid = rule->dst_uid;
insert_rule(pol, nrule);
}
}
return res;
}
static ssize_t handle_policy_update(struct file *file,
const char __user *ubuf, size_t len)
{
@ -128,7 +159,7 @@ static ssize_t handle_policy_update(struct file *file,
goto out_free_rule;
}
hash_add(pol->rules, &rule->next, __kuid_val(rule->src_uid));
insert_rule(pol, rule);
p = end + 1;
continue;
@ -137,6 +168,11 @@ static ssize_t handle_policy_update(struct file *file,
goto out_free_buf;
}
err = verify_ruleset(pol);
/* bogus policy falls through after fixing it up */
if (err && err != -EINVAL)
goto out_free_buf;
/*
* Everything looks good, apply the policy and release the old one.
* What we really want here is an xchg() wrapper for RCU, but since that

4
tools/testing/selftests/safesetid/safesetid-test.c
View File

@ -144,7 +144,9 @@ static void write_policies(void)
{
static char *policy_str =
"1:2\n"
"1:3\n";
"1:3\n"
"2:2\n"
"3:3\n";
ssize_t written;
int fd;