rcu: Fix buffer overflow in print_cpu_stall_info()

The rcuc-starvation output from print_cpu_stall_info() might overflow the
buffer if there is a huge difference in jiffies difference.  The situation
might seem improbable, but computers sometimes get very confused about
time, which can result in full-sized integers, and, in this case,
buffer overflow.

Also, the unsigned jiffies difference is printed using %ld, which is
normally for signed integers.  This is intentional for debugging purposes,
but it is not obvious from the code.

This commit therefore changes sprintf() to snprintf() and adds a
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@ancud.ru>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 8a2edf6a1ef5815481ecc1dac58693f2b2d84d23..460efecd077be13940a94fe5a06197f25e6921a9 100644 (file)
--- 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 values indicate a probable 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)],