seccomp: add "seccomp" syscall

This adds the new "seccomp" syscall with both an "operation" and "flags"
parameter for future expansion. The third argument is a pointer value,
used with the SECCOMP_SET_MODE_FILTER operation. Currently, flags must
be 0. This is functionally equivalent to prctl(PR_SET_SECCOMP, ...).

In addition to the TSYNC flag later in this patch series, there is a
non-zero chance that this syscall could be used for configuring a fixed
argument area for seccomp-tracer-aware processes to pass syscall arguments
in the future. Hence, the use of "seccomp" not simply "seccomp_add_filter"
for this syscall. Additionally, this syscall uses operation, flags,
and user pointer for arguments because strictly passing arguments via
a user pointer would mean seccomp itself would be unable to trivially
filter the seccomp syscall itself.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>

2 files changed, 53 insertions, 5 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 05cac2c2eca1..f0652578af75 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -18,6 +18,7 @@
 #include <linux/compat.h>
 #include <linux/sched.h>
 #include <linux/seccomp.h>
+#include <linux/syscalls.h>
 
 /* #define SECCOMP_DEBUG 1 */
 
@@ -314,7 +315,7 @@ free_prog:
  *
  * Returns 0 on success and non-zero otherwise.
  */
-static long seccomp_attach_user_filter(char __user *user_filter)
+static long seccomp_attach_user_filter(const char __user *user_filter)
 {
         struct sock_fprog fprog;
         long ret = -EFAULT;
@@ -517,6 +518,7 @@ out:
 #ifdef CONFIG_SECCOMP_FILTER
 /**
  * seccomp_set_mode_filter: internal function for setting seccomp filter
+ * @flags:  flags to change filter behavior
  * @filter: struct sock_fprog containing filter
  *
  * This function may be called repeatedly to install additional filters.
@@ -527,11 +529,16 @@ out:
  *
  * Returns 0 on success or -EINVAL on failure.
  */
-static long seccomp_set_mode_filter(char __user *filter)
+static long seccomp_set_mode_filter(unsigned int flags,
+                                    const char __user *filter)
 {
         const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
         long ret = -EINVAL;
 
+        /* Validate flags. */
+        if (flags != 0)
+                goto out;
+
         if (!seccomp_may_assign_mode(seccomp_mode))
                 goto out;
 
@@ -544,12 +551,35 @@ out:
         return ret;
 }
 #else
-static inline long seccomp_set_mode_filter(char __user *filter)
+static inline long seccomp_set_mode_filter(unsigned int flags,
+                                           const char __user *filter)
 {
         return -EINVAL;
 }
 #endif
 
+/* Common entry point for both prctl and syscall. */
+static long do_seccomp(unsigned int op, unsigned int flags,
+                       const char __user *uargs)
+{
+        switch (op) {
+        case SECCOMP_SET_MODE_STRICT:
+                if (flags != 0 || uargs != NULL)
+                        return -EINVAL;
+                return seccomp_set_mode_strict();
+        case SECCOMP_SET_MODE_FILTER:
+                return seccomp_set_mode_filter(flags, uargs);
+        default:
+                return -EINVAL;
+        }
+}
+
+SYSCALL_DEFINE3(seccomp, unsigned int, op, unsigned int, flags,
+                         const char __user *, uargs)
+{
+        return do_seccomp(op, flags, uargs);
+}
+
 /**
  * prctl_set_seccomp: configures current->seccomp.mode
  * @seccomp_mode: requested mode to use
@@ -559,12 +589,27 @@ static inline long seccomp_set_mode_filter(char __user *filter)
  */
 long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
 {
+        unsigned int op;
+        char __user *uargs;
+
         switch (seccomp_mode) {
         case SECCOMP_MODE_STRICT:
-                return seccomp_set_mode_strict();
+                op = SECCOMP_SET_MODE_STRICT;
+                /*
+                 * Setting strict mode through prctl always ignored filter,
+                 * so make sure it is always NULL here to pass the internal
+                 * check in do_seccomp().
+                 */
+                uargs = NULL;
+                break;
         case SECCOMP_MODE_FILTER:
-                return seccomp_set_mode_filter(filter);
+                op = SECCOMP_SET_MODE_FILTER;
+                uargs = filter;
+                break;
         default:
                 return -EINVAL;
         }
+
+        /* prctl interface doesn't have flags, so they are always zero. */
+        return do_seccomp(op, 0, uargs);
 }
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 36441b51b5df..2904a2105914 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -213,3 +213,6 @@ cond_syscall(compat_sys_open_by_handle_at);
 
 /* compare kernel pointers */
 cond_syscall(sys_kcmp);
+
+/* operate on Secure Computing state */
+cond_syscall(sys_seccomp);