Re: [PATCH] sched/psi: Bounds-check state iterator against NR_PSI_STATES

[Johannes Weiner]

On Fri, May 20, 2022 at 09:58:26AM -0700, Kees Cook wrote:
> GCC 12 cannot tell that "t" will be bounded by NR_PSI_STATES, which could
> lead to walking off the end of the tasks array, which is NR_PSI_STATES in
> size. Explicitly bounds-check "t" as part of the loop.
> 
> In file included from ../kernel/sched/build_utility.c:97:
> ../kernel/sched/psi.c: In function 'psi_group_change':
> ../kernel/sched/psi.c:730:38: warning: array subscript 32 is above array bounds of 'unsigned int[5]' [-Warray-bounds]
>   730 |                         groupc->tasks[t]++;
>       |                         ~~~~~~~~~~~~~^~~
> In file included from ../include/linux/psi.h:6,
>                  from ../kernel/sched/build_utility.c:36:
> ../include/linux/psi_types.h:84:22: note: while referencing 'tasks'
>    84 |         unsigned int tasks[NR_PSI_TASK_COUNTS];
>       |                      ^~~~~
> ../kernel/sched/psi.c:730:38: warning: array subscript 32 is above array bounds of 'unsigned int[5]' [-Warray-bounds]
>   730 |                         groupc->tasks[t]++;
>       |                         ~~~~~~~~~~~~~^~~
> ../include/linux/psi_types.h:84:22: note: while referencing 'tasks'
>    84 |         unsigned int tasks[NR_PSI_TASK_COUNTS];
>       |                      ^~~~~
> 
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  kernel/sched/psi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index a337f3e35997..827f16a79936 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -725,7 +725,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
>  		}
>  	}
>  
> -	for (t = 0; set; set &= ~(1 << t), t++)
> +	for (t = 0; set && t < ARRAY_SIZE(groupc->tasks); set &= ~(1 << t), t++)
>  		if (set & (1 << t))
>  			groupc->tasks[t]++;

This is a very hot path, it runs for every nested cgroup on every task
switch, wakeup and sleep. We should avoid unnecessary instructions and
branches if we can help it at all.

Does the below patch address the warning for you? I can't test it
myself, because I'm not getting it with gcc version 12.1.0. It's also
odd that it didn't warn you about the loop over `clear' a few lines
up, which ostensibly has the same "problem".

---

diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
index c7fe7c089718..113861343733 100644
--- a/include/linux/psi_types.h
+++ b/include/linux/psi_types.h
@@ -41,6 +41,7 @@ enum psi_task_count {
 #define TSK_RUNNING	(1 << NR_RUNNING)
 #define TSK_ONCPU	(1 << NR_ONCPU)
 #define TSK_MEMSTALL_RUNNING	(1 << NR_MEMSTALL_RUNNING)
+#define TSK_MASK	((1U << NR_PSI_TASK_COUNTS) - 1)
 
 /* Resources that workloads could be stalled on */
 enum psi_res {
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index a4fa3aadfcba..fb7fd40af337 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -804,6 +804,8 @@ void psi_task_change(struct task_struct *task, int clear, int set)
 	void *iter = NULL;
 	u64 now;
 
+	WARN_ON_ONCE((clear|set) & ~TSK_MASK);
+
 	if (!task->pid)
 		return;