[RFC PATCH] capability: add capable_or to test for multiple caps with exactly one audit message

[RFC PATCH] capability: add capable_or to test for multiple caps with exactly one audit message

[Christian Göttsche]

WARNING: only compile tested and just to test the water

Add the interface `capable_or()` as an alternative to or multiple
`capable()` calls, like `capable_or(CAP_SYS_NICE, CAP_SYS_ADMIN)`
instead of `capable(CAP_SYS_NICE) || capable(CAP_SYS_ADMIN)`.
`capable_or()` will in particular generate exactly one audit message,
either for the left most capability in effect or, if the task has none,
the first one.
This is especially helpful with regard to SELinux, where each audit
message about a not allowed capability will create an avc denial.
Using this function with the least invasive capability as left most
argument (e.g. CAP_SYS_NICE before CAP_SYS_ADMIN) enables policy writers
to only allow the least invasive one and SELinux domains pass this check
with only capability:sys_nice or capability:sys_admin allowed without
any avc denial message.

Fixes: 94c4b4fd25e6 ("block: Check ADMIN before NICE for IOPRIO_CLASS_RT")
---
 block/ioprio.c             |  2 +-
 include/linux/capability.h |  6 +++++
 kernel/capability.c        | 46 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/block/ioprio.c b/block/ioprio.c
index 0e4ff245f2bf..d00466d46da3 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -69,7 +69,7 @@ int ioprio_check_cap(int ioprio)
 
 	switch (class) {
 		case IOPRIO_CLASS_RT:
-			if (!capable(CAP_SYS_NICE) && !capable(CAP_SYS_ADMIN))
+			if (!capable_or(CAP_SYS_NICE, CAP_SYS_ADMIN))
 				return -EPERM;
 			fallthrough;
 			/* rt has prio field too */
diff --git a/include/linux/capability.h b/include/linux/capability.h
index 65efb74c3585..b9943b44cdc8 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -207,6 +207,8 @@ extern bool has_ns_capability(struct task_struct *t,
 extern bool has_capability_noaudit(struct task_struct *t, int cap);
 extern bool has_ns_capability_noaudit(struct task_struct *t,
 				      struct user_namespace *ns, int cap);
+#define capable_or(...) _capable_or((sizeof((int[]){__VA_ARGS__}) / sizeof(int)), __VA_ARGS__)
+extern bool _capable_or(int count, ...);
 extern bool capable(int cap);
 extern bool ns_capable(struct user_namespace *ns, int cap);
 extern bool ns_capable_noaudit(struct user_namespace *ns, int cap);
@@ -230,6 +232,10 @@ static inline bool has_ns_capability_noaudit(struct task_struct *t,
 {
 	return true;
 }
+static inline bool capable_or(int first_cap, ...)
+{
+	return true;
+}
 static inline bool capable(int cap)
 {
 	return true;
diff --git a/kernel/capability.c b/kernel/capability.c
index 46a361dde042..0ee03ae4c1b2 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -434,6 +434,52 @@ bool ns_capable_setid(struct user_namespace *ns, int cap)
 }
 EXPORT_SYMBOL(ns_capable_setid);
 
+/**
+ * _capable_or - Determine if the current task has one of multiple superior capabilities in effect
+ * @cap: The capabilities to be tested for
+ *
+ * Return true if the current task has at least one of the given superior capabilities currently
+ * available for use, false if not.
+ *
+ * In contrast to or'ing capable() this call will create exactly one audit message, either for the
+ * left most capability in effect or (if the task has none of the tested capabilities) the first
+ * capabilit in the test list.
+ *
+ * This sets PF_SUPERPRIV on the task if the capability is available on the
+ * assumption that it's about to be used.
+ */
+bool _capable_or(int count, ...)
+{
+	va_list args;
+	int cap, first_cap, i;
+
+	BUG_ON(count < 1);
+	BUG_ON(count > CAP_LAST_CAP);
+
+	va_start(args, count);
+
+	for (i = 0; i < count; i++) {
+		int ret;
+
+		cap = va_arg(args, int);
+		if (i == 0)
+			first_cap = cap;
+
+		rcu_read_lock();
+		ret = security_capable(current_cred(), &init_user_ns, cap, CAP_OPT_NOAUDIT);
+		rcu_read_unlock();
+
+		if (ret == 0)
+			goto out;
+	}
+
+	cap = first_cap;
+
+out:
+	va_end(args);
+	return ns_capable(&init_user_ns, cap);
+}
+
 /**
  * capable - Determine if the current task has a superior capability in effect
  * @cap: The capability to be tested for
-- 
2.33.1