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authorJann Horn <jannh@google.com>2019-04-11 13:12:43 -0700
committerMicah Morton <mortonm@chromium.org>2019-07-15 08:07:51 -0700
commit4f72123da579655855301b591535a1415224f123 (patch)
tree6b9ca3a8a23eb20b41591819ee7fef3b04f207b4
parentfbd9acb2dc2aa55902c48a83f157082849209fba (diff)
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LSM: SafeSetID: verify transitive constrainedness
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>
-rw-r--r--security/safesetid/securityfs.c38
-rw-r--r--tools/testing/selftests/safesetid/safesetid-test.c4
2 files changed, 40 insertions, 2 deletions
diff --git a/security/safesetid/securityfs.c b/security/safesetid/securityfs.c
index 997b403c6255..d568e17dd773 100644
--- a/security/safesetid/securityfs.c
+++ b/security/safesetid/securityfs.c
@@ -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 @@ out_free_rule:
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
diff --git a/tools/testing/selftests/safesetid/safesetid-test.c b/tools/testing/selftests/safesetid/safesetid-test.c
index 4f03813d1911..8f40c6ecdad1 100644
--- a/tools/testing/selftests/safesetid/safesetid-test.c
+++ b/tools/testing/selftests/safesetid/safesetid-test.c
@@ -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;