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 <oleg@tv-sign.ru>
Cc: Alexey Dobriyan <adobriyan@sw.ru>
Cc: Michael Kerrisk <mtk.manpages@googlemail.com>
Cc: Pavel Emelyanov <xemul@sw.ru>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Toyo Abe <toyoa@mvista.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

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

1 files changed, 3 insertions, 14 deletions

diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index ce26896..022c9c3 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -982,20 +982,9 @@ sys_clock_getres(const clockid_t which_clock, struct timespec __user *tp)
 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