x86: Enable HAVE_ARCH_SECCOMP_FILTER

Enable support for seccomp filter on x86:
- syscall_get_arch()
- syscall_get_arguments()
- syscall_rollback()
- syscall_set_return_value()
- SIGSYS siginfo_t support
- secure_computing is called from a ptrace_event()-safe context
- secure_computing return value is checked (see below).

SECCOMP_RET_TRACE and SECCOMP_RET_TRAP may result in seccomp needing to
skip a system call without killing the process.  This is done by
returning a non-zero (-1) value from secure_computing.  This change
makes x86 respect that return value.

To ensure that minimal kernel code is exposed, a non-zero return value
results in an immediate return to user space (with an invalid syscall
number).

Signed-off-by: Will Drewry <wad@chromium.org>
Reviewed-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Eric Paris <eparis@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>

v18: rebase and tweaked change description, acked-by
v17: added reviewed by and rebased
v..: all rebases since original introduction.
Signed-off-by: James Morris <james.l.morris@oracle.com>

arch/x86/Kconfig

@@ -82,6 +82,7 @@ config X86
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select GENERIC_IOMAP
 	select DCACHE_WORD_ACCESS if !DEBUG_PAGEALLOC
+	select HAVE_ARCH_SECCOMP_FILTER
 
 config INSTRUCTION_DECODER
 	def_bool (KPROBES || PERF_EVENTS)

arch/x86/kernel/ptrace.c

@@ -1480,7 +1480,11 @@ long syscall_trace_enter(struct pt_regs *regs)
 		regs->flags |= X86_EFLAGS_TF;
 
 	/* do the secure computing check first */
-	secure_computing(regs->orig_ax);
+	if (secure_computing(regs->orig_ax)) {
+		/* seccomp failures shouldn't expose any additional code. */
+		ret = -1L;
+		goto out;
+	}
 
 	if (unlikely(test_thread_flag(TIF_SYSCALL_EMU)))
 		ret = -1L;
@@ -1505,6 +1509,7 @@ long syscall_trace_enter(struct pt_regs *regs)
 				    regs->dx, regs->r10);
 #endif
 
+out:
 	return ret ?: regs->orig_ax;
 }