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@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 3.5.x and later
---
Documentation/security/Yama.txt | 14 ++++++------
include/linux/security.h | 2 -
security/yama/yama_lsm.c | 41 +++++++++++++++++++++++++++++++++++++++
3 files changed, 48 insertions(+), 9 deletions(-)
diff --git a/Documentation/security/Yama.txt b/Documentation/security/Yama.txt
index e369de2..dd908cf 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.
diff --git a/include/linux/security.h b/include/linux/security.h
index 4e5a73c..3dea6a9 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -1242,8 +1242,6 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
* Check that the @parent process has sufficient permission to trace the
* current process before allowing the current process to present itself
* to the @parent process for tracing.
- * The parent process will still have to undergo the ptrace_access_check
- * checks before it is allowed to trace this one.
* @parent contains the task_struct structure for debugger process.
* Return 0 if permission is granted.
* @capget:
diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index 83554ee..d51b7c7 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -290,10 +290,51 @@ static int yama_ptrace_access_check(struct task_struct *child,
return rc;
}
+/**
+ * yama_ptrace_traceme - validate PTRACE_TRACEME calls
+ * @parent: task that will become the ptracer of the current task
+ *
+ * Returns 0 if following the ptrace is allowed, -ve on error.
+ */
+static int yama_ptrace_traceme(struct task_struct *parent)
+{
+ int rc;
+
+ /* If standard caps disallows it, so does Yama. We should
+ * only tighten restrictions further.
+ */
+ rc = cap_ptrace_traceme(parent);
+ if (rc)
+ return rc;
+
+ /* Only disallow PTRACE_TRACEME on more aggressive settings. */
+ switch (ptrace_scope) {
+ case YAMA_SCOPE_CAPABILITY:
+ if (!ns_capable(task_user_ns(parent), CAP_SYS_PTRACE))
+ rc = -EPERM;
+ break;
+ case YAMA_SCOPE_NO_ATTACH:
+ rc = -EPERM;
+ break;
+ }
+
+ if (rc) {
+ char name[sizeof(current->comm)];
+ printk_ratelimited(KERN_NOTICE
+ "ptraceme of pid %d was attempted by: %s (pid %d)\n",
+ current->pid,
+ get_task_comm(name, parent),
+ parent->pid);
+ }
+
+ return rc;
+}
+
static struct security_operations yama_ops = {
.name = "yama",
.ptrace_access_check = yama_ptrace_access_check,
+ .ptrace_traceme = yama_ptrace_traceme,
.task_prctl = yama_task_prctl,
.task_free = yama_task_free,
};
--
1.7.0.4
--
Kees Cook
Chrome OS Security
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