seccomp: Migrate to use SYSCALL_WORK flag

On architectures using the generic syscall entry code the architecture
independent syscall work is moved to flags in thread_info::syscall_work.
This removes architecture dependencies and frees up TIF bits.

Define SYSCALL_WORK_SECCOMP, use it in the generic entry code and convert
the code which uses the TIF specific helper functions to use the new
*_syscall_work() helpers which either resolve to the new mode for users of
the generic entry code or to the TIF based functions for the other
architectures.
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20201116174206.2639648-5-krisman@collabora.com

include/asm-generic/syscall.h

@@ -135,7 +135,7 @@ void syscall_set_arguments(struct task_struct *task, struct pt_regs *regs,
  * Returns the AUDIT_ARCH_* based on the system call convention in use.
  *
  * It's only valid to call this when @task is stopped on entry to a system
- * call, due to %TIF_SYSCALL_TRACE, %TIF_SYSCALL_AUDIT, or %TIF_SECCOMP.
+ * call, due to %TIF_SYSCALL_TRACE, %TIF_SYSCALL_AUDIT, or %SYSCALL_WORK_SECCOMP.
  *
  * Architectures which permit CONFIG_HAVE_ARCH_SECCOMP_FILTER must
  * provide an implementation of this.

include/linux/entry-common.h

@@ -21,10 +21,6 @@
 # define _TIF_SYSCALL_TRACEPOINT	(0)
 #endif
 
-#ifndef _TIF_SECCOMP
-# define _TIF_SECCOMP			(0)
-#endif
-
 #ifndef _TIF_SYSCALL_AUDIT
 # define _TIF_SYSCALL_AUDIT		(0)
 #endif
@@ -49,7 +45,7 @@
 #endif
 
 #define SYSCALL_ENTER_WORK						\
-	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | _TIF_SECCOMP |	\
+	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT  |			\
 	 _TIF_SYSCALL_TRACEPOINT | _TIF_SYSCALL_EMU |			\
 	 ARCH_SYSCALL_ENTER_WORK)
 
@@ -64,7 +60,7 @@
 	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT |			\
 	 _TIF_SYSCALL_TRACEPOINT | ARCH_SYSCALL_EXIT_WORK)
 
-#define SYSCALL_WORK_ENTER	(0)
+#define SYSCALL_WORK_ENTER	(SYSCALL_WORK_SECCOMP)
 #define SYSCALL_WORK_EXIT	(0)
 
 /*

include/linux/seccomp.h

@@ -42,7 +42,7 @@ struct seccomp {
 extern int __secure_computing(const struct seccomp_data *sd);
 static inline int secure_computing(void)
 {
-	if (unlikely(test_thread_flag(TIF_SECCOMP)))
+	if (unlikely(test_syscall_work(SECCOMP)))
 		return  __secure_computing(NULL);
 	return 0;
 }

include/linux/thread_info.h

@@ -35,6 +35,12 @@ enum {
 	GOOD_STACK,
 };
 
+enum syscall_work_bit {
+	SYSCALL_WORK_BIT_SECCOMP,
+};
+
+#define SYSCALL_WORK_SECCOMP		BIT(SYSCALL_WORK_BIT_SECCOMP)
+
 #include <asm/thread_info.h>
 
 #ifdef __KERNEL__

kernel/entry/common.c

@@ -54,7 +54,7 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
 	}
 
 	/* Do seccomp after ptrace, to catch any tracer changes. */
-	if (ti_work & _TIF_SECCOMP) {
+	if (work & SYSCALL_WORK_SECCOMP) {
 		ret = __secure_computing(NULL);
 		if (ret == -1L)
 			return ret;

kernel/fork.c

@@ -1625,7 +1625,7 @@ static void copy_seccomp(struct task_struct *p)
 	 * to manually enable the seccomp thread flag here.
 	 */
 	if (p->seccomp.mode != SECCOMP_MODE_DISABLED)
-		set_tsk_thread_flag(p, TIF_SECCOMP);
+		set_task_syscall_work(p, SECCOMP);
 #endif
 }
 

kernel/seccomp.c

@@ -356,14 +356,14 @@ static inline void seccomp_assign_mode(struct task_struct *task,
 
 	task->seccomp.mode = seccomp_mode;
 	/*
-	 * Make sure TIF_SECCOMP cannot be set before the mode (and
+	 * Make sure SYSCALL_WORK_SECCOMP cannot be set before the mode (and
 	 * filter) is set.
 	 */
 	smp_mb__before_atomic();
 	/* Assume default seccomp processes want spec flaw mitigation. */
 	if ((flags & SECCOMP_FILTER_FLAG_SPEC_ALLOW) == 0)
 		arch_seccomp_spec_mitigate(task);
-	set_tsk_thread_flag(task, TIF_SECCOMP);
+	set_task_syscall_work(task, SECCOMP);
 }
 
 #ifdef CONFIG_SECCOMP_FILTER
@@ -929,7 +929,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 
 	/*
 	 * Make sure that any changes to mode from another thread have
-	 * been seen after TIF_SECCOMP was seen.
+	 * been seen after SYSCALL_WORK_SECCOMP was seen.
 	 */
 	rmb();