[PATCH 1/2] arm64: Add seccomp support

[AKASHI Takahiro]

secure_computing() should always be called first in syscall_trace(), and
if it returns non-zero, we should stop further handling. Then that system
call may eventually fail, be trapped or the process itself be killed
depending on loaded rules.

This patch also defines specific system call numbers, __NR_seccomp_*,
solely used by secure_computing() for seccomp mode 1 (only read, write, exit
and sigreturn are allowd).

Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
---
 arch/arm64/Kconfig               |   17 +++++++++++++++++
 arch/arm64/include/asm/seccomp.h |   28 ++++++++++++++++++++++++++++
 arch/arm64/include/asm/unistd.h  |    3 +++
 arch/arm64/kernel/entry.S        |    4 ++++
 arch/arm64/kernel/ptrace.c       |    5 +++++
 5 files changed, 57 insertions(+)
 create mode 100644 arch/arm64/include/asm/seccomp.h

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index a21455e..a0102f7 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -27,6 +27,7 @@ config ARM64
 	select HARDIRQS_SW_RESEND
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_ARCH_JUMP_LABEL
+	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_DEBUG_BUGVERBOSE
 	select HAVE_DEBUG_KMEMLEAK
@@ -222,6 +223,22 @@ config HAVE_ARCH_TRANSPARENT_HUGEPAGE
 
 source "mm/Kconfig"
 
+config SECCOMP
+	def_bool y
+	prompt "Enable seccomp to safely compute untrusted bytecode"
+	---help---
+	  This kernel feature is useful for number crunching applications
+	  that may need to compute untrusted bytecode during their
+	  execution. By using pipes or other transports made available to
+	  the process as file descriptors supporting the read/write
+	  syscalls, it's possible to isolate those applications in
+	  their own address space using seccomp. Once seccomp is
+	  enabled via prctl(PR_SET_SECCOMP), it cannot be disabled
+	  and the task is only allowed to execute a few safe syscalls
+	  defined by each seccomp mode.
+
+	  If unsure, say Y. Only embedded should say N here.
+
 config XEN_DOM0
 	def_bool y
 	depends on XEN
diff --git a/arch/arm64/include/asm/seccomp.h b/arch/arm64/include/asm/seccomp.h
new file mode 100644
index 0000000..3482155
--- /dev/null
+++ b/arch/arm64/include/asm/seccomp.h
@@ -0,0 +1,28 @@
+/*
+ * arch/arm64/include/asm/seccomp.h
+ *
+ * Copyright (C) 2014 Linaro Limited
+ * Author: AKASHI Takahiro <takahiro.akashi at linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+#ifndef _ASM_SECCOMP_H
+#define _ASM_SECCOMP_H
+
+#include <asm/unistd.h>
+
+#ifdef CONFIG_COMPAT
+#define __NR_seccomp_read_32		__NR_compat_read
+#define __NR_seccomp_write_32		__NR_compat_write
+#define __NR_seccomp_exit_32		__NR_compat_exit
+#define __NR_seccomp_sigreturn_32	__NR_compat_sigreturn
+#endif /* CONFIG_COMPAT */
+
+#define __NR_seccomp_read		__NR_read
+#define __NR_seccomp_write		__NR_write
+#define __NR_seccomp_exit		__NR_exit
+#define __NR_seccomp_sigreturn		__NR_rt_sigreturn
+
+#endif /* _ASM_SECCOMP_H */
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 4a09fdb..05f2db3 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h
@@ -30,6 +30,9 @@
  * Compat syscall numbers used by the AArch64 kernel.
  */
 #define __NR_compat_restart_syscall	0
+#define __NR_compat_exit		1
+#define __NR_compat_read		3
+#define __NR_compat_write		4
 #define __NR_compat_sigreturn		119
 #define __NR_compat_rt_sigreturn	173
 
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 96c2d03..55d4e6c 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -651,6 +651,10 @@ __sys_trace:
 	mov	x1, sp
 	mov	w0, #0				// trace entry
 	bl	syscall_trace
+#ifdef CONFIG_SECCOMP
+	cmp	w0, #0				// check seccomp result
+	b.lt	ret_to_user			// -1 means 'rejected'
+#endif
 	adr	lr, __sys_trace_return		// return address
 	uxtw	scno, w0			// syscall number (possibly new)
 	mov	x1, sp				// pointer to regs
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 8cdba09..3bfe398 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -26,6 +26,7 @@
 #include <linux/smp.h>
 #include <linux/ptrace.h>
 #include <linux/user.h>
+#include <linux/seccomp.h>
 #include <linux/security.h>
 #include <linux/init.h>
 #include <linux/signal.h>
@@ -1064,6 +1065,10 @@ asmlinkage int syscall_trace(int dir, struct pt_regs *regs)
 {
 	unsigned long saved_reg;
 
+	if (!dir && secure_computing((int)regs->syscallno))
+		/* seccomp failures shouldn't expose any additional code. */
+		return -1;
+
 	if (is_compat_task()) {
 		/* AArch32 uses ip (r12) for scratch */
 		saved_reg = regs->regs[12];
-- 
1.7.9.5