Re: [PATCH v3] rcu: Fix buffer overlow in print_cpu_stall_info()

[Steven Rostedt]

On Mon,  1 Apr 2024 22:43:15 +0300
Nikita Kiryushin <kiryushin@xxxxxxxx> wrote:

> rcuc info output in print_cpu_stall_info() contains
> posiible buffer overflow in the case of huge jiffies
> difference. The situation seems improbable, but, buffer
> overflow, still.
> 
> Also, unsigned jiffies difference printed as (signed)
> %ld. This is intentional for debugging purposes, but
> it is not obvious from the code.
> 
> Change sprintf to snprintf and add clarifying comment
> about intention of %ld format.
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: 245a62982502 ("rcu: Dump rcuc kthread status for CPUs not reporting quiescent state")
> Signed-off-by: Nikita Kiryushin <kiryushin@xxxxxxxx>
> ---
> v3: Change intention comment wording as
> Steven Rostedt <rostedt@xxxxxxxxxxx> suggested
> v2: Remove signed to unsigned print format change as
> Paul E. McKenney <paulmck@xxxxxxxxxx> suggested, add format
> intention clarification comment
>  kernel/rcu/tree_stall.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 5d666428546b..320440b8384e 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -504,7 +504,8 @@ static void print_cpu_stall_info(int cpu)
>  			rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
>  	rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
>  	if (rcuc_starved)
> -		sprintf(buf, " rcuc=%ld jiffies(starved)", j);
> +		/* Print signed value, as negative means it is likely a bug */
> +		snprintf(buf, sizeof(buf), " rcuc=%ld jiffies(starved)", j);
>  	pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
>  	       cpu,
>  	       "O."[!!cpu_online(cpu)],


Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

-- Steve