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

[Paul E. McKenney]

On Mon, Apr 01, 2024 at 04:03:12PM -0400, Steven Rostedt wrote:
> 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>

Queued for v6.10, thank you both!

							Thanx, Paul