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

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

[Max R. P. Grossmann]

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@max.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

Re: [PATCH] time: POSIX CPU timers: Ensure that variable is initialized

[Thomas Gleixner]

On Mon, 8 Jan 2018, Max R. P. Grossmann wrote:

> 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.

Sure the explanation makes sense, though you should have mentioned that
_ALL_ current users call this function with a valid clockid.

I'll amend the changelog.


> Signed-off-by: Max R. P. Grossmann <m@max.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
> 
> 

[tip:timers/core] posix-cpu-timers: Make set_process_cpu_timer() more robust

[tip-bot for Max R. P. Grossmann]

Commit-ID:  a9445e47d897054876b8f43e46dc5a3eca2b844d
Gitweb:     https://git.kernel.org/tip/a9445e47d897054876b8f43e46dc5a3eca2b844d
Author:     Max R. P. Grossmann <m@max.pm>
AuthorDate: Mon, 8 Jan 2018 20:01:57 +0100
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 14 Jan 2018 20:50:59 +0100

posix-cpu-timers: Make set_process_cpu_timer() more robust

Because the return value of cpu_timer_sample_group() is not checked,
compilers and static checkers can legitimately warn about a potential use
of the uninitialized variable 'now'. This is not a runtime issue as all call
sites hand in valid clock ids.

Also cpu_timer_sample_group() is invoked unconditionally even when the
result is not used because *oldval is NULL.

Make the invocation conditional and check the return value.

[ tglx: Massage changelog ]

Signed-off-by: Max R. P. Grossmann <m@max.pm>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: john.stultz@linaro.org
Link: https://lkml.kernel.org/r/20180108190157.10048-1-m@max.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 cef79ca..ec9f5da 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