linux-kernel - [PATCH] time: POSIX CPU timers: Ensure that variable is initialized

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Message-Id: <20180108190157.10048-1-m@max.pm>
Date:   Mon,  8 Jan 2018 20:01:57 +0100
From:   "Max R. P. Grossmann" <m@....pm>
To:     linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, john.stultz@...aro.org,
        "Max R. P. Grossmann" <m@....pm>
Subject: [PATCH] time: POSIX CPU timers: Ensure that variable is initialized

If cpu_timer_sample_group returns -EINVAL, it will not have written into
*sample. Checking for cpu_timer_sample_group's return value precludes the
potential use of an uninitialized value of now in the following block.
Given an invalid clock_idx, the previous code could otherwise overwrite
*oldval in an undefined manner. This is now prevented. We also exploit
short-circuiting of && to sample the timer only if the result will
actually be used to update *oldval.

Signed-off-by: Max R. P. Grossmann <m@....pm>
---
 kernel/time/posix-cpu-timers.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 1f27887aa194..e54638be6e19 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1189,9 +1189,8 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
 	u64 now;
 
 	WARN_ON_ONCE(clock_idx == CPUCLOCK_SCHED);
-	cpu_timer_sample_group(clock_idx, tsk, &now);
 
-	if (oldval) {
+	if (oldval && cpu_timer_sample_group(clock_idx, tsk, &now) != -EINVAL) {
 		/*
 		 * We are setting itimer. The *oldval is absolute and we update
 		 * it to be relative, *newval argument is relative and we update
-- 
2.15.1