nospec: Allow getting/setting on non-current task

Adjust arch_prctl_get/set_spec_ctrl() to operate on tasks other than
current.

This is needed both for /proc/$pid/status queries and for seccomp (since
thread-syncing can trigger seccomp in non-current threads).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/cpu/bugs.c

@@ -530,31 +530,35 @@ static void ssb_select_mitigation()
 
 #undef pr_fmt
 
-static int ssb_prctl_set(unsigned long ctrl)
+static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
 {
-	bool rds = !!test_tsk_thread_flag(current, TIF_RDS);
+	bool rds = !!test_tsk_thread_flag(task, TIF_RDS);
 
 	if (ssb_mode != SPEC_STORE_BYPASS_PRCTL)
 		return -ENXIO;
 
 	if (ctrl == PR_SPEC_ENABLE)
-		clear_tsk_thread_flag(current, TIF_RDS);
+		clear_tsk_thread_flag(task, TIF_RDS);
 	else
-		set_tsk_thread_flag(current, TIF_RDS);
+		set_tsk_thread_flag(task, TIF_RDS);
 
-	if (rds != !!test_tsk_thread_flag(current, TIF_RDS))
+	/*
+	 * If being set on non-current task, delay setting the CPU
+	 * mitigation until it is next scheduled.
+	 */
+	if (task == current && rds != !!test_tsk_thread_flag(task, TIF_RDS))
 		speculative_store_bypass_update();
 
 	return 0;
 }
 
-static int ssb_prctl_get(void)
+static int ssb_prctl_get(struct task_struct *task)
 {
 	switch (ssb_mode) {
 	case SPEC_STORE_BYPASS_DISABLE:
 		return PR_SPEC_DISABLE;
 	case SPEC_STORE_BYPASS_PRCTL:
-		if (test_tsk_thread_flag(current, TIF_RDS))
+		if (test_tsk_thread_flag(task, TIF_RDS))
 			return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
 		return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
 	default:
@@ -564,24 +568,25 @@ static int ssb_prctl_get(void)
 	}
 }
 
-int arch_prctl_spec_ctrl_set(unsigned long which, unsigned long ctrl)
+int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
+			     unsigned long ctrl)
 {
 	if (ctrl != PR_SPEC_ENABLE && ctrl != PR_SPEC_DISABLE)
 		return -ERANGE;
 
 	switch (which) {
 	case PR_SPEC_STORE_BYPASS:
-		return ssb_prctl_set(ctrl);
+		return ssb_prctl_set(task, ctrl);
 	default:
 		return -ENODEV;
 	}
 }
 
-int arch_prctl_spec_ctrl_get(unsigned long which)
+int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
 {
 	switch (which) {
 	case PR_SPEC_STORE_BYPASS:
-		return ssb_prctl_get();
+		return ssb_prctl_get(task);
 	default:
 		return -ENODEV;
 	}

include/linux/nospec.h

@@ -7,6 +7,8 @@
 #define _LINUX_NOSPEC_H
 #include <asm/barrier.h>
 
+struct task_struct;
+
 /**
  * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise
  * @index: array element index
@@ -57,7 +59,8 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 })
 
 /* Speculation control prctl */
-int arch_prctl_spec_ctrl_get(unsigned long which);
-int arch_prctl_spec_ctrl_set(unsigned long which, unsigned long ctrl);
+int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which);
+int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
+			     unsigned long ctrl);
 
 #endif /* _LINUX_NOSPEC_H */

kernel/sys.c

@@ -2244,12 +2244,13 @@ static int propagate_has_child_subreaper(struct task_struct *p, void *data)
 	return 1;
 }
 
-int __weak arch_prctl_spec_ctrl_get(unsigned long which)
+int __weak arch_prctl_spec_ctrl_get(struct task_struct *t, unsigned long which)
 {
 	return -EINVAL;
 }
 
-int __weak arch_prctl_spec_ctrl_set(unsigned long which, unsigned long ctrl)
+int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
+				    unsigned long ctrl)
 {
 	return -EINVAL;
 }
@@ -2465,12 +2466,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_GET_SPECULATION_CTRL:
 		if (arg3 || arg4 || arg5)
 			return -EINVAL;
-		error = arch_prctl_spec_ctrl_get(arg2);
+		error = arch_prctl_spec_ctrl_get(me, arg2);
 		break;
 	case PR_SET_SPECULATION_CTRL:
 		if (arg4 || arg5)
 			return -EINVAL;
-		error = arch_prctl_spec_ctrl_set(arg2, arg3);
+		error = arch_prctl_spec_ctrl_set(me, arg2, arg3);
 		break;
 	default:
 		error = -EINVAL;