[v1] security: add trace event for cap_capable

[Jordan Rome]

In cases where we want a stable way to observe/trace
cap_capable (e.g. protection from inlining and API updates)
add a tracepoint that passes:
- The credentials used
- The user namespace which needs the capability
- The user namespace that actually has the capability (if one exists)
- The capability to check for
- Bitmask of options defined in include/linux/security.h
- The return value of the check

Signed-off-by: Jordan Rome <linux at jordanrome.com>
---
 MAINTAINERS                       |  1 +
 include/trace/events/capability.h | 58 +++++++++++++++++++++++++++++++
 security/commoncap.c              | 21 +++++++----
 3 files changed, 74 insertions(+), 6 deletions(-)
 create mode 100644 include/trace/events/capability.h

diff --git a/MAINTAINERS b/MAINTAINERS
index cc40a9d9b8cd..210e9076c858 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4994,6 +4994,7 @@ M:	Serge Hallyn <serge at hallyn.com>
 L:	linux-security-module at vger.kernel.org
 S:	Supported
 F:	include/linux/capability.h
+F:	include/trace/events/capability.h
 F:	include/uapi/linux/capability.h
 F:	kernel/capability.c
 F:	security/commoncap.c
diff --git a/include/trace/events/capability.h b/include/trace/events/capability.h
new file mode 100644
index 000000000000..092b8e77063a
--- /dev/null
+++ b/include/trace/events/capability.h
@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM capability
+
+#if !defined(_TRACE_CAPABILITY_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_CAPABILITY_H
+
+#include <linux/cred.h>
+#include <linux/tracepoint.h>
+#include <linux/user_namespace.h>
+
+/**
+ * capable - called after it's determined if a task has a particular
+ * effective capability
+ *
+ * @cred: The credentials used
+ * @targ_ns:  The user namespace which needs the capability
+ * @capable_ns:  The user namespace that actually has the capability
+ *               if ret is 0 otherwise this will be NULL
+ * @cap: The capability to check for
+ * @opts: Bitmask of options defined in include/linux/security.h
+ * @ret: The return value of the check: 0 if it does, -ve if it does not
+ *
+ * Allows to trace calls to cap_capable in commoncap.c
+ */
+TRACE_EVENT(capable,
+
+	TP_PROTO(const struct cred *cred, struct user_namespace *targ_ns,
+		struct user_namespace *capable_ns, int cap, unsigned int opts, int ret),
+
+	TP_ARGS(cred, targ_ns, capable_ns, cap, opts, ret),
+
+	TP_STRUCT__entry(
+		__field(const struct cred *, cred)
+		__field(struct user_namespace *, targ_ns)
+		__field(struct user_namespace *, capable_ns)
+		__field(int, cap)
+		__field(unsigned int, opts)
+		__field(int, ret)
+	),
+
+	TP_fast_assign(
+		__entry->cred       = cred;
+		__entry->targ_ns    = targ_ns;
+		__entry->capable_ns = capable_ns;
+		__entry->cap        = cap;
+		__entry->opts       = opts;
+		__entry->ret        = ret;
+	),
+
+	TP_printk("cap %d, opts %u, ret %d",
+		__entry->cap, __entry->opts, __entry->ret)
+);
+
+#endif /* _TRACE_CAPABILITY_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
diff --git a/security/commoncap.c b/security/commoncap.c
index 162d96b3a676..675d40fbaa77 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -27,6 +27,9 @@
 #include <linux/mnt_idmapping.h>
 #include <uapi/linux/lsm.h>

+#define CREATE_TRACE_POINTS
+#include <trace/events/capability.h>
+
 /*
  * If a non-root user executes a setuid-root binary in
  * !secure(SECURE_NOROOT) mode, then we raise capabilities.
@@ -68,6 +71,7 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		int cap, unsigned int opts)
 {
 	struct user_namespace *ns = targ_ns;
+	int ret = 0;

 	/* See if cred has the capability in the target user namespace
 	 * by examining the target user namespace and all of the target
@@ -75,22 +79,26 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 	 */
 	for (;;) {
 		/* Do we have the necessary capabilities? */
-		if (ns == cred->user_ns)
-			return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
+		if (ns == cred->user_ns) {
+			ret = cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
+			break;
+		}

 		/*
 		 * If we're already at a lower level than we're looking for,
 		 * we're done searching.
 		 */
-		if (ns->level <= cred->user_ns->level)
-			return -EPERM;
+		if (ns->level <= cred->user_ns->level) {
+			ret = -EPERM;
+			break;
+		}

 		/*
 		 * The owner of the user namespace in the parent of the
 		 * user namespace has all caps.
 		 */
 		if ((ns->parent == cred->user_ns) && uid_eq(ns->owner, cred->euid))
-			return 0;
+			break;

 		/*
 		 * If you have a capability in a parent user ns, then you have
@@ -99,7 +107,8 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		ns = ns->parent;
 	}

-	/* We never get here */
+	trace_capable(cred, targ_ns, ret == 0 ? ns : NULL, cap, opts, ret);
+	return ret;
 }

 /**
--
2.43.5