Yama: higher restrictions should block PTRACE_TRACEME

The higher ptrace restriction levels should be blocking even
PTRACE_TRACEME requests. The comments in the LSM documentation are
misleading about when the checks happen (the parent does not go through
security_ptrace_access_check() on a PTRACE_TRACEME call).

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org # 3.5.x and later
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 7 insertions, 7 deletions

diff --git a/Documentation/security/Yama.txt b/Documentation/security/Yama.txt
index e369de2d48cd..dd908cf64ecf 100644
--- a/Documentation/security/Yama.txt
+++ b/Documentation/security/Yama.txt
@@ -46,14 +46,13 @@ restrictions, it can call prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)
 so that any otherwise allowed process (even those in external pid namespaces)
 may attach.
 
-These restrictions do not change how ptrace via PTRACE_TRACEME operates.
-
-The sysctl settings are:
+The sysctl settings (writable only with CAP_SYS_PTRACE) are:
 
 0 - classic ptrace permissions: a process can PTRACE_ATTACH to any other
     process running under the same uid, as long as it is dumpable (i.e.
     did not transition uids, start privileged, or have called
-    prctl(PR_SET_DUMPABLE...) already).
+    prctl(PR_SET_DUMPABLE...) already). Similarly, PTRACE_TRACEME is
+    unchanged.
 
 1 - restricted ptrace: a process must have a predefined relationship
     with the inferior it wants to call PTRACE_ATTACH on. By default,
@@ -61,12 +60,13 @@ The sysctl settings are:
     classic criteria is also met. To change the relationship, an
     inferior can call prctl(PR_SET_PTRACER, debugger, ...) to declare
     an allowed debugger PID to call PTRACE_ATTACH on the inferior.
+    Using PTRACE_TRACEME is unchanged.
 
 2 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace
-    with PTRACE_ATTACH.
+    with PTRACE_ATTACH, or through children calling PTRACE_TRACEME.
 
-3 - no attach: no processes may use ptrace with PTRACE_ATTACH. Once set,
-    this sysctl cannot be changed to a lower value.
+3 - no attach: no processes may use ptrace with PTRACE_ATTACH nor via
+    PTRACE_TRACEME. Once set, this sysctl value cannot be changed.
 
 The original children-only logic was based on the restrictions in grsecurity.