ARM: SECCOMP support

Signed-off-by: Nicolas Pitre <[e-mail]>

arch/arm/Kconfig

···
 	  However, if the CPU data cache is using a write-allocate mode,
 	  this option is unlikely to provide any performance gain.
 
+config SECCOMP
+	bool
+	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.
+
 config CC_STACKPROTECTOR
 	bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
 	help

arch/arm/include/asm/seccomp.h

···
+#ifndef _ASM_ARM_SECCOMP_H
+#define _ASM_ARM_SECCOMP_H
+
+#include <linux/unistd.h>
+
+#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_ARM_SECCOMP_H */

arch/arm/include/asm/thread_info.h

···
 #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
 #define TIF_FREEZE		19
 #define TIF_RESTORE_SIGMASK	20
+#define TIF_SECCOMP		21
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
···
 #define _TIF_USING_IWMMXT	(1 << TIF_USING_IWMMXT)
 #define _TIF_FREEZE		(1 << TIF_FREEZE)
 #define _TIF_RESTORE_SIGMASK	(1 << TIF_RESTORE_SIGMASK)
+#define _TIF_SECCOMP		(1 << TIF_SECCOMP)
 
 /*
  * Change these and you break ASM code in entry-common.S

arch/arm/kernel/entry-common.S

···
 
 	get_thread_info tsk
 	adr	tbl, sys_call_table		@ load syscall table pointer
-	ldr	ip, [tsk, #TI_FLAGS]		@ check for syscall tracing
 
 #if defined(CONFIG_OABI_COMPAT)
 	/*
···
 	eor	scno, scno, #__NR_SYSCALL_BASE	@ check OS number
 #endif
 
+	ldr	r10, [tsk, #TI_FLAGS]		@ check for syscall tracing
 	stmdb	sp!, {r4, r5}			@ push fifth and sixth args
-	tst	ip, #_TIF_SYSCALL_TRACE		@ are we tracing syscalls?
+
+#ifdef CONFIG_SECCOMP
+	tst	r10, #_TIF_SECCOMP
+	beq	1f
+	mov	r0, scno
+	bl	__secure_computing	
+	add	r0, sp, #S_R0 + S_OFF		@ pointer to regs
+	ldmia	r0, {r0 - r3}			@ have to reload r0 - r3
+1:
+#endif
+
+	tst	r10, #_TIF_SYSCALL_TRACE		@ are we tracing syscalls?
 	bne	__sys_trace
 
 	cmp	scno, #NR_syscalls		@ check upper syscall limit