security: add trace event for cap_capable

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 of the resource being accessed
- The user namespace in which the credential provides the
capability to access the targeted resource
- The capability to check for
- The return value of the check

Signed-off-by: Jordan Rome <linux@jordanrome.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Link: https://lore.kernel.org/r/20241204155911.1817092-1-linux@jordanrome.com
Signed-off-by: Serge Hallyn <sergeh@kernel.org>

1 files changed, 41 insertions, 13 deletions

diff --git a/security/commoncap.c b/security/commoncap.c
index 3d103069903b..bdd7603fabb8 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.
@@ -50,24 +53,24 @@ static void warn_setuid_and_fcaps_mixed(const char *fname)
 }
 
 /**
- * cap_capable - Determine whether a task has a particular effective capability
+ * cap_capable_helper - Determine whether a task has a particular effective
+ * capability.
  * @cred: The credentials to use
- * @targ_ns:  The user namespace in which we need the capability
+ * @target_ns:  The user namespace of the resource being accessed
+ * @cred_ns:  The user namespace of the credentials
  * @cap: The capability to check for
- * @opts: Bitmask of options defined in include/linux/security.h
  *
  * Determine whether the nominated task has the specified capability amongst
  * its effective set, returning 0 if it does, -ve if it does not.
  *
- * NOTE WELL: cap_has_capability() cannot be used like the kernel's capable()
- * and has_capability() functions.  That is, it has the reverse semantics:
- * cap_has_capability() returns 0 when a task has a capability, but the
- * kernel's capable() and has_capability() returns 1 for this case.
+ * See cap_capable for more details.
  */
-int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
-		int cap, unsigned int opts)
+static inline int cap_capable_helper(const struct cred *cred,
+				     struct user_namespace *target_ns,
+				     const struct user_namespace *cred_ns,
+				     int cap)
 {
-	struct user_namespace *ns = targ_ns;
+	struct user_namespace *ns = target_ns;
 
 	/* See if cred has the capability in the target user namespace
 	 * by examining the target user namespace and all of the target
@@ -75,21 +78,21 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 	 */
 	for (;;) {
 		/* Do we have the necessary capabilities? */
-		if (ns == cred->user_ns)
+		if (likely(ns == cred_ns))
 			return cap_raised(cred->cap_effective, cap) ? 0 : -EPERM;
 
 		/*
 		 * If we're already at a lower level than we're looking for,
 		 * we're done searching.
 		 */
-		if (ns->level <= cred->user_ns->level)
+		if (ns->level <= cred_ns->level)
 			return -EPERM;
 
 		/* 
 		 * 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))
+		if ((ns->parent == cred_ns) && uid_eq(ns->owner, cred->euid))
 			return 0;
 
 		/*
@@ -103,6 +106,31 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 }
 
 /**
+ * cap_capable - Determine whether a task has a particular effective capability
+ * @cred: The credentials to use
+ * @target_ns:  The user namespace of the resource being accessed
+ * @cap: The capability to check for
+ * @opts: Bitmask of options defined in include/linux/security.h (unused)
+ *
+ * Determine whether the nominated task has the specified capability amongst
+ * its effective set, returning 0 if it does, -ve if it does not.
+ *
+ * NOTE WELL: cap_has_capability() cannot be used like the kernel's capable()
+ * and has_capability() functions.  That is, it has the reverse semantics:
+ * cap_has_capability() returns 0 when a task has a capability, but the
+ * kernel's capable() and has_capability() returns 1 for this case.
+ */
+int cap_capable(const struct cred *cred, struct user_namespace *target_ns,
+		int cap, unsigned int opts)
+{
+	const struct user_namespace *cred_ns = cred->user_ns;
+	int ret = cap_capable_helper(cred, target_ns, cred_ns, cap);
+
+	trace_cap_capable(cred, target_ns, cred_ns, cap, ret);
+	return ret;
+}
+
+/**
  * cap_settime - Determine whether the current process may set the system clock
  * @ts: The time to set
  * @tz: The timezone to set