m68k: Add kernel seccomp support

Add secure_computing() call to syscall_trace_enter to actually
filter system calls.

Add necessary arch Kconfig options, define TIF_SECCOMP trace
flag and provide basic seccomp filter support in asm/syscall.h

syscall_get_nr currently uses the syscall nr stored in orig_d0
because we change d0 to a default return code before starting a
syscall trace. This may be inconsistent with syscall_rollback
copying orig_d0 to d0 (which we never check upon return from
trace). We use d0 for the return code from syscall_trace_enter
in entry.S currently, and could perhaps expand that to store
a new syscall number returned by the seccomp filter before
executing the syscall. This clearly needs some discussion.

seccomp_bpf self test on ARAnyM passes 81 out of 94 tests.

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20230112035529.13521-3-schmitzmic@gmail.com
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

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

@@ -14,7 +14,7 @@
     |     hexagon: | TODO |
     |        ia64: | TODO |
     |   loongarch: |  ok  |
-    |        m68k: | TODO |
+    |        m68k: |  ok  |
     |  microblaze: | TODO |
     |        mips: |  ok  |
     |       nios2: | TODO |

arch/m68k/Kconfig

@@ -18,6 +18,8 @@ config M68K
 	select GENERIC_CPU_DEVICES
 	select GENERIC_IOMAP
 	select GENERIC_IRQ_SHOW
+	select HAVE_ARCH_SECCOMP
+	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_DEBUG_BUGVERBOSE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED

arch/m68k/include/asm/seccomp.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _ASM_SECCOMP_H
+#define _ASM_SECCOMP_H
+
+#include <asm-generic/seccomp.h>
+
+#define SECCOMP_ARCH_NATIVE		AUDIT_ARCH_M68K
+#define SECCOMP_ARCH_NATIVE_NR		NR_syscalls
+#define SECCOMP_ARCH_NATIVE_NAME	"m68k"
+
+#endif /* _ASM_SECCOMP_H */

arch/m68k/include/asm/syscall.h

@@ -4,6 +4,63 @@
 
 #include <uapi/linux/audit.h>
 
+#include <asm/unistd.h>
+
+extern const unsigned long sys_call_table[];
+
+static inline int syscall_get_nr(struct task_struct *task,
+				 struct pt_regs *regs)
+{
+	return regs->orig_d0;
+}
+
+static inline void syscall_rollback(struct task_struct *task,
+				    struct pt_regs *regs)
+{
+	regs->d0 = regs->orig_d0;
+}
+
+static inline long syscall_get_error(struct task_struct *task,
+				     struct pt_regs *regs)
+{
+	unsigned long error = regs->d0;
+
+	return IS_ERR_VALUE(error) ? error : 0;
+}
+
+static inline long syscall_get_return_value(struct task_struct *task,
+					    struct pt_regs *regs)
+{
+	return regs->d0;
+}
+
+static inline void syscall_set_return_value(struct task_struct *task,
+					    struct pt_regs *regs,
+					    int error, long val)
+{
+	regs->d0 = (long)error ?: val;
+}
+
+static inline void syscall_get_arguments(struct task_struct *task,
+					 struct pt_regs *regs,
+					 unsigned long *args)
+{
+	args[0] = regs->orig_d0;
+	args++;
+
+	memcpy(args, &regs->d1, 5 * sizeof(args[0]));
+}
+
+static inline void syscall_set_arguments(struct task_struct *task,
+					 struct pt_regs *regs,
+					 unsigned long *args)
+{
+	regs->orig_d0 = args[0];
+	args++;
+
+	memcpy(&regs->d1, args, 5 * sizeof(args[0]));
+}
+
 static inline int syscall_get_arch(struct task_struct *task)
 {
 	return AUDIT_ARCH_M68K;

arch/m68k/include/asm/thread_info.h

@@ -61,6 +61,7 @@ static inline struct thread_info *current_thread_info(void)
 #define TIF_NOTIFY_RESUME	5	/* callback before returning to user */
 #define TIF_SIGPENDING		6	/* signal pending */
 #define TIF_NEED_RESCHED	7	/* rescheduling necessary */
+#define TIF_SECCOMP		13	/* seccomp syscall filtering active */
 #define TIF_DELAYED_TRACE	14	/* single step a syscall */
 #define TIF_SYSCALL_TRACE	15	/* syscall trace active */
 #define TIF_MEMDIE		16	/* is terminating due to OOM killer */
@@ -69,6 +70,7 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
+#define _TIF_SECCOMP		(1 << TIF_SECCOMP)
 #define _TIF_DELAYED_TRACE	(1 << TIF_DELAYED_TRACE)
 #define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
 #define _TIF_MEMDIE		(1 << TIF_MEMDIE)

arch/m68k/kernel/entry.S

@@ -214,6 +214,9 @@ ENTRY(system_call)
 	| syscall trace?
 	tstb	%a1@(TINFO_FLAGS+2)
 	jmi	do_trace_entry
+	| seccomp filter active?
+	btst	#5,%a1@(TINFO_FLAGS+2)
+	bnes	do_trace_entry
 	cmpl	#NR_syscalls,%d0
 	jcc	badsys
 syscall:

arch/m68k/kernel/ptrace.c

@@ -21,7 +21,7 @@
 #include <linux/signal.h>
 #include <linux/regset.h>
 #include <linux/elf.h>
-
+#include <linux/seccomp.h>
 #include <linux/uaccess.h>
 #include <asm/page.h>
 #include <asm/processor.h>
@@ -278,6 +278,10 @@ asmlinkage int syscall_trace_enter(void)
 
 	if (test_thread_flag(TIF_SYSCALL_TRACE))
 		ret = ptrace_report_syscall_entry(task_pt_regs(current));
+
+	if (secure_computing() == -1)
+		return -1;
+
 	return ret;
 }