[PATCH v2 1/4] seccomp: Add sysctl to display available actions

[Tyler Hicks]

This patch creates a read-only sysctl containing an ordered list of
seccomp actions that the kernel supports. The ordering, from left to
right, is the lowest action value (kill) to the highest action value
(allow). Currently, a read of the sysctl file would return "kill trap
errno trace allow". The contents of this sysctl file can be useful for
userspace code as well as the system administrator.

The path to the sysctl is:

  /proc/sys/kernel/seccomp/actions_avail

libseccomp and other userspace code can easily determine which actions
the current kernel supports. The set of actions supported by the current
kernel may be different than the set of action macros found in kernel
headers that were installed where the userspace code was built.

In addition, this sysctl will allow system administrators to know which
actions are supported by the kernel and make it easier to configure
exactly what seccomp logs through the audit subsystem. Support for this
level of logging configuration will come in a future patch.

Signed-off-by: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
---
 kernel/seccomp.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index f7ce79a..919ad9f 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -16,10 +16,12 @@
 #include <linux/atomic.h>
 #include <linux/audit.h>
 #include <linux/compat.h>
+#include <linux/kmemleak.h>
 #include <linux/sched.h>
 #include <linux/seccomp.h>
 #include <linux/slab.h>
 #include <linux/syscalls.h>
+#include <linux/sysctl.h>
 
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
 #include <asm/syscall.h>
@@ -905,3 +907,51 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
 	return ret;
 }
 #endif
+
+#ifdef CONFIG_SYSCTL
+
+#define SECCOMP_RET_KILL_NAME		"kill"
+#define SECCOMP_RET_TRAP_NAME		"trap"
+#define SECCOMP_RET_ERRNO_NAME		"errno"
+#define SECCOMP_RET_TRACE_NAME		"trace"
+#define SECCOMP_RET_ALLOW_NAME		"allow"
+
+static char seccomp_actions_avail[] = SECCOMP_RET_KILL_NAME	" "
+				      SECCOMP_RET_TRAP_NAME	" "
+				      SECCOMP_RET_ERRNO_NAME	" "
+				      SECCOMP_RET_TRACE_NAME	" "
+				      SECCOMP_RET_ALLOW_NAME;
+
+static struct ctl_path seccomp_sysctl_path[] = {
+	{ .procname = "kernel", },
+	{ .procname = "seccomp", },
+	{ }
+};
+
+static struct ctl_table seccomp_sysctl_table[] = {
+	{
+		.procname	= "actions_avail",
+		.data		= &seccomp_actions_avail,
+		.maxlen		= sizeof(seccomp_actions_avail),
+		.mode		= 0444,
+		.proc_handler	= proc_dostring,
+	},
+	{ }
+};
+
+static int __init seccomp_sysctl_init(void)
+{
+	struct ctl_table_header *hdr;
+
+	hdr = register_sysctl_paths(seccomp_sysctl_path, seccomp_sysctl_table);
+	kmemleak_not_leak(hdr);
+	return 0;
+}
+
+#else /* CONFIG_SYSCTL */
+
+static __init int seccomp_sysctl_init(void) { return 0; }
+
+#endif /* CONFIG_SYSCTL */
+
+device_initcall(seccomp_sysctl_init)
-- 
2.7.4