xtensa: add seccomp support

Add SECCOMP to xtensa Kconfig, select HAVE_ARCH_SECCOMP_FILTER, add
TIF_SECCOMP and call secure_computing from do_syscall_trace_enter.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Documentation/features/seccomp/seccomp-filter/arch-support.txt

@@ -30,5 +30,5 @@
     |          um: |  ok  |
     |   unicore32: | TODO |
     |         x86: |  ok  |
-    |      xtensa: | TODO |
+    |      xtensa: |  ok  |
     -----------------------

arch/xtensa/Kconfig

@@ -24,6 +24,7 @@ config XTENSA
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
 	select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL
+	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_DEBUG_KMEMLEAK
@@ -217,6 +218,20 @@ config HOTPLUG_CPU
 
 	  Say N if you want to disable CPU hotplug.
 
+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 FAST_SYSCALL_XTENSA
 	bool "Enable fast atomic syscalls"
 	default n

arch/xtensa/include/asm/Kbuild

@@ -7,4 +7,5 @@ generic-y += mcs_spinlock.h
 generic-y += param.h
 generic-y += qrwlock.h
 generic-y += qspinlock.h
+generic-y += seccomp.h
 generic-y += user.h

arch/xtensa/include/asm/thread_info.h

@@ -112,6 +112,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_NOTIFY_RESUME	7	/* callback before returning to user */
 #define TIF_DB_DISABLED		8	/* debug trap disabled for syscall */
 #define TIF_SYSCALL_AUDIT	9	/* syscall auditing active */
+#define TIF_SECCOMP		10	/* secure computing */
 
 #define _TIF_SYSCALL_TRACE	(1<<TIF_SYSCALL_TRACE)
 #define _TIF_SIGPENDING		(1<<TIF_SIGPENDING)
@@ -119,9 +120,11 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_SINGLESTEP		(1<<TIF_SINGLESTEP)
 #define _TIF_SYSCALL_TRACEPOINT	(1<<TIF_SYSCALL_TRACEPOINT)
 #define _TIF_SYSCALL_AUDIT	(1<<TIF_SYSCALL_AUDIT)
+#define _TIF_SECCOMP		(1<<TIF_SECCOMP)
 
 #define _TIF_WORK_MASK		(_TIF_SYSCALL_TRACE | _TIF_SINGLESTEP | \
-				 _TIF_SYSCALL_TRACEPOINT | _TIF_SYSCALL_AUDIT)
+				 _TIF_SYSCALL_TRACEPOINT | \
+				 _TIF_SYSCALL_AUDIT | _TIF_SECCOMP)
 
 #define THREAD_SIZE KERNEL_STACK_SIZE
 #define THREAD_SIZE_ORDER (KERNEL_STACK_SHIFT - PAGE_SHIFT)

arch/xtensa/kernel/ptrace.c

@@ -22,6 +22,7 @@
 #include <linux/regset.h>
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
+#include <linux/seccomp.h>
 #include <linux/security.h>
 #include <linux/signal.h>
 #include <linux/smp.h>
@@ -559,7 +560,8 @@ int do_syscall_trace_enter(struct pt_regs *regs)
 		return 0;
 	}
 
-	if (regs->syscall == NO_SYSCALL) {
+	if (regs->syscall == NO_SYSCALL ||
+	    secure_computing() == -1) {
 		do_syscall_trace_leave(regs);
 		return 0;
 	}