basic: Fix capability_ambient_set_apply for kernels < 4.3

kkuehlz · keszybz · commit 7ea4392f1e44 · 2020-03-29T21:11:25.000+02:00
systemd#14133 made capability_ambient_set_apply() acquire capabilities that were explicitly asked for and drop all others. This change means the function is called even with an empty capability set, opening up a code path for users without ambient capabilities to call this function. This function will error with EINVAL out on kernels < 4.3 because PR_CAP_AMBIENT is not understood. This turns capability_ambient_set_apply() into a noop for kernels < 4.3 Fixes systemd#15225
diff --git a/src/basic/capability-util.c b/src/basic/capability-util.c
@@ -107,6 +107,10 @@ int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
         unsigned long i;
         int r;
 
+        /* Check that we can use PR_CAP_AMBIENT or quit early. */
+        if (!ambient_capabilities_supported())
+                return 0;
+
         /* Add the capabilities to the ambient set. */
 
         if (also_inherit) {
