[PATCH v3 5/5] capabilities: don't let ptrace borrow CAP_SETFCAP

[Josef Bacik <josef@toxicpanda.com>]

Tasks in a user namespace created by a task with CAP_SETFCAP may map uid
0 of the parent and write file capabilities that are honoured there.
That is the one privilege they keep outside of their namespace.  setns()
no longer lets anybody in who doesn't have it, but the ptrace checks
don't know about it: for a target in another user namespace
cap_ptrace_access_check() is satisfied with CAP_SYS_PTRACE over the
target's namespace, which the owner rule hands to any task with the
right euid.

  task A: uid 0, full caps          task B: uid 0, no CAP_SETFCAP
  unshare(CLONE_NEWUSER)
                                    ptrace(PTRACE_ATTACH, A)
                                      uids match, euid == ns->owner
                                    make A write "0 0 1" to its uid_map,
                                    or set file capabilities
                                      A is entitled -> allowed

The same goes for /proc/<pid>/mem, process_vm_writev() and pidfd_getfd().

Require for PTRACE_MODE_ATTACH and PTRACE_TRACEME what setns() requires
to get into the target's namespace: CAP_SETFCAP of the tracer has to
reach the root users of ancestors that tasks in the namespace can write
file capabilities for, or the tracer has CAP_SYS_PTRACE over the topmost
of those ancestors, which gives control of tasks there that have
CAP_SETFCAP anyway.  As in setns(), root users stored through an
idmapped mount are not considered.  A tracer in the target's namespace
got in with as much and always passes.

Targets in the initial namespace, in namespaces created without
CAP_SETFCAP (what unprivileged users create) and in namespaces that
don't map the root user of an ancestor (the usual container) are not
affected.  What remains is a tracer that gave up CAP_SETFCAP and
CAP_SYS_PTRACE and a target in a namespace that maps the root user of
an ancestor and was created by a task that didn't give it up, which the
tracer can't setns() into either.  PTRACE_MODE_READ is unchanged.

Fixes: db2e718a4798 ("capabilities: require CAP_SETFCAP to map uid 0")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 security/commoncap.c | 45 +++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 41 insertions(+), 4 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 1a419a4a872c..af7564b38b67 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -254,14 +254,41 @@ int cap_settime(const struct timespec64 *ts, const struct timezone *tz)
 	return 0;
 }
 
+/*
+ * Tasks in a user namespace have CAP_SETFCAP that can count in ancestors of
+ * the namespace, see cap_setfcap_level().  That is a privilege outside of the
+ * namespace, so capabilities over the namespace are not enough to take
+ * control of such a task.
+ */
+static bool cap_covers_setfcap(const struct cred *cred,
+			       const struct cred *child_cred)
+{
+	struct user_namespace *ns = child_cred->user_ns, *top;
+
+	/* setns() only lets in tasks whose CAP_SETFCAP reaches as far. */
+	if (cred->user_ns == ns)
+		return true;
+
+	top = setfcap_top(ns);
+	if (!top)
+		return true;
+
+	/* CAP_SYS_PTRACE up there gives control of tasks with CAP_SETFCAP. */
+	return cred_setfcap_reaches(cred, ns->parent, top->level) ||
+	       !cap_capable(cred, top, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
+}
+
 /**
  * cap_ptrace_access_check - Determine whether the current process may access
  *			   another
  * @child: The process to be accessed
  * @mode: The mode of attachment.
  *
- * If we are in the same or an ancestor user_ns and have all the target
- * task's capabilities, then ptrace access is allowed.
+ * For PTRACE_MODE_ATTACH, if the target task may make use of CAP_SETFCAP in
+ * an ancestor of its user_ns that our CAP_SETFCAP or CAP_SYS_PTRACE doesn't
+ * reach, then ptrace access is denied.
+ * Otherwise, if we are in the same or an ancestor user_ns and have all the
+ * target task's capabilities, then ptrace access is allowed.
  * If we have the ptrace capability to the target user_ns, then ptrace
  * access is allowed.
  * Else denied.
@@ -282,11 +309,15 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
 		caller_caps = &cred->cap_effective;
 	else
 		caller_caps = &cred->cap_permitted;
+	if ((mode & PTRACE_MODE_ATTACH) &&
+	    !cap_covers_setfcap(cred, child_cred))
+		goto deny;
 	if (cred->user_ns == child_cred->user_ns &&
 	    cap_issubset(child_cred->cap_permitted, *caller_caps))
 		goto out;
 	if (ns_capable(child_cred->user_ns, CAP_SYS_PTRACE))
 		goto out;
+deny:
 	ret = -EPERM;
 out:
 	rcu_read_unlock();
@@ -297,8 +328,11 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
  * cap_ptrace_traceme - Determine whether another process may trace the current
  * @parent: The task proposed to be the tracer
  *
- * If parent is in the same or an ancestor user_ns and has all current's
- * capabilities, then ptrace access is allowed.
+ * If current may make use of CAP_SETFCAP in an ancestor of its user_ns that
+ * parent's CAP_SETFCAP or CAP_SYS_PTRACE doesn't reach, then ptrace access
+ * is denied.
+ * Otherwise, if parent is in the same or an ancestor user_ns and has all
+ * current's capabilities, then ptrace access is allowed.
  * If parent has the ptrace capability to current's user_ns, then ptrace
  * access is allowed.
  * Else denied.
@@ -314,11 +348,14 @@ int cap_ptrace_traceme(struct task_struct *parent)
 	rcu_read_lock();
 	cred = __task_cred(parent);
 	child_cred = current_cred();
+	if (!cap_covers_setfcap(cred, child_cred))
+		goto deny;
 	if (cred->user_ns == child_cred->user_ns &&
 	    cap_issubset(child_cred->cap_permitted, cred->cap_permitted))
 		goto out;
 	if (has_ns_capability(parent, child_cred->user_ns, CAP_SYS_PTRACE))
 		goto out;
+deny:
 	ret = -EPERM;
 out:
 	rcu_read_unlock();

-- 
2.55.0