Bug 1294286 - Filter clock IDs in clock_getres sandbox rule. r=gcp

The clockid_t type on Linux has a space of values with encode a pid and
refer to various measures of another process's CPU usage; clock_getres
would, thereby, allow probing whether other processes exist.  This is
a relatively small information leak into the sandboxes, but there's no
reason to allow it.

Differential Revision: https://phabricator.services.mozilla.com/D54081

--HG--
extra : moz-landing-system : lando

security/sandbox/linux/SandboxFilter.cpp

@@ -452,6 +452,7 @@ class SandboxPolicyCommon : public SandboxPolicyBase {
     switch (sysno) {
         // Timekeeping
       case __NR_clock_nanosleep:
+      case __NR_clock_getres:
       case __NR_clock_gettime: {
         // clockid_t can encode a pid or tid to monitor another
         // process or thread's CPU usage (see CPUCLOCK_PID and related
@@ -1175,7 +1176,6 @@ class ContentSandboxPolicy : public SandboxPolicyCommon {
         return Allow();
 
       CASES_FOR_getrlimit:
-      case __NR_clock_getres:
       CASES_FOR_getresuid:
       CASES_FOR_getresgid:
         return Allow();