hrtimer: fix *rmtp handling in hrtimer_nanosleep()

Spotted by Pavel Emelyanov and Alexey Dobriyan.

hrtimer_nanosleep() sets restart_block->arg1 = rmtp, but this rmtp points to
the local variable which lives in the caller's stack frame. This means that
if sys_restart_syscall() actually happens and it is interrupted as well, we
don't update the user-space variable, but write into the already dead stack
frame.

Introduced by commit 04c227140fed77587432667a574b14736a06dd7f
hrtimer: Rework hrtimer_nanosleep to make sys_compat_nanosleep easier

Change the callers to pass "__user *rmtp" to hrtimer_nanosleep(), and change
hrtimer_nanosleep() to use copy_to_user() to actually update *rmtp.

Small problem remains. man 2 nanosleep states that *rtmp should be written if
nanosleep() was interrupted (it says nothing whether it is OK to update *rmtp
if nanosleep returns 0), but (with or without this patch) we can dirty *rem
even if nanosleep() returns 0.

NOTE: this patch doesn't change compat_sys_nanosleep(), because it has other
bugs. Fixed by the next patch.

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Michael Kerrisk <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Toyo Abe <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>

include/linux/hrtimer.h | 2 -
kernel/hrtimer.c | 51 +++++++++++++++++++++++++-----------------------
kernel/posix-timers.c | 14 +------------
3 files changed, 30 insertions(+), 37 deletions(-)

include/linux/hrtimer.h

···
 
 /* Precise sleep: */
 extern long hrtimer_nanosleep(struct timespec *rqtp,
-			      struct timespec *rmtp,
+			      struct timespec __user *rmtp,
 			      const enum hrtimer_mode mode,
 			      const clockid_t clockid);
 extern long hrtimer_nanosleep_restart(struct restart_block *restart_block);

kernel/hrtimer.c

···
 	return t->task == NULL;
 }
 
+static int update_rmtp(struct hrtimer *timer, struct timespec __user *rmtp)
+{
+	struct timespec rmt;
+	ktime_t rem;
+
+	rem = ktime_sub(timer->expires, timer->base->get_time());
+	if (rem.tv64 <= 0)
+		return 0;
+	rmt = ktime_to_timespec(rem);
+
+	if (copy_to_user(rmtp, &rmt, sizeof(*rmtp)))
+		return -EFAULT;
+
+	return 1;
+}
+
 long __sched hrtimer_nanosleep_restart(struct restart_block *restart)
 {
 	struct hrtimer_sleeper t;
-	struct timespec *rmtp;
-	ktime_t time;
+	struct timespec __user  *rmtp;
 
 	restart->fn = do_no_restart_syscall;
 
···
 	if (do_nanosleep(&t, HRTIMER_MODE_ABS))
 		return 0;
 
-	rmtp = (struct timespec *)restart->arg1;
+	rmtp = (struct timespec __user *)restart->arg1;
 	if (rmtp) {
-		time = ktime_sub(t.timer.expires, t.timer.base->get_time());
-		if (time.tv64 <= 0)
-			return 0;
-		*rmtp = ktime_to_timespec(time);
+		int ret = update_rmtp(&t.timer, rmtp);
+		if (ret <= 0)
+			return ret;
 	}
 
 	restart->fn = hrtimer_nanosleep_restart;
···
 	return -ERESTART_RESTARTBLOCK;
 }
 
-long hrtimer_nanosleep(struct timespec *rqtp, struct timespec *rmtp,
+long hrtimer_nanosleep(struct timespec *rqtp, struct timespec __user *rmtp,
 		       const enum hrtimer_mode mode, const clockid_t clockid)
 {
 	struct restart_block *restart;
 	struct hrtimer_sleeper t;
-	ktime_t rem;
 
 	hrtimer_init(&t.timer, clockid, mode);
 	t.timer.expires = timespec_to_ktime(*rqtp);
···
 		return -ERESTARTNOHAND;
 
 	if (rmtp) {
-		rem = ktime_sub(t.timer.expires, t.timer.base->get_time());
-		if (rem.tv64 <= 0)
-			return 0;
-		*rmtp = ktime_to_timespec(rem);
+		int ret = update_rmtp(&t.timer, rmtp);
+		if (ret <= 0)
+			return ret;
 	}
 
 	restart = &current_thread_info()->restart_block;
···
 asmlinkage long
 sys_nanosleep(struct timespec __user *rqtp, struct timespec __user *rmtp)
 {
-	struct timespec tu, rmt;
-	int ret;
+	struct timespec tu;
 
 	if (copy_from_user(&tu, rqtp, sizeof(tu)))
 		return -EFAULT;
···
 	if (!timespec_valid(&tu))
 		return -EINVAL;
 
-	ret = hrtimer_nanosleep(&tu, rmtp ? &rmt : NULL, HRTIMER_MODE_REL,
-				CLOCK_MONOTONIC);
-
-	if (ret && rmtp) {
-		if (copy_to_user(rmtp, &rmt, sizeof(*rmtp)))
-			return -EFAULT;
-	}
-
-	return ret;
+	return hrtimer_nanosleep(&tu, rmtp, HRTIMER_MODE_REL, CLOCK_MONOTONIC);
 }
 
 /*

kernel/posix-timers.c

···
 static int common_nsleep(const clockid_t which_clock, int flags,
 			 struct timespec *tsave, struct timespec __user *rmtp)
 {
-	struct timespec rmt;
-	int ret;
-
-	ret = hrtimer_nanosleep(tsave, rmtp ? &rmt : NULL,
-				flags & TIMER_ABSTIME ?
-				HRTIMER_MODE_ABS : HRTIMER_MODE_REL,
-				which_clock);
-
-	if (ret && rmtp) {
-		if (copy_to_user(rmtp, &rmt, sizeof(*rmtp)))
-			return -EFAULT;
-	}
-
-	return ret;
+	return hrtimer_nanosleep(tsave, rmtp, flags & TIMER_ABSTIME ?
+				 HRTIMER_MODE_ABS : HRTIMER_MODE_REL,
+				 which_clock);
 }
 
 asmlinkage long