[PATCH] x86, sched: Prevent divisions by zero in frequency invariant accounting

From: Giovanni Gherdovich <ggherdovich@suse.cz>
To: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>
Cc: x86@kernel.org, linux-pm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@suse.de>, Len Brown <lenb@kernel.org>,
	Giovanni Gherdovich <ggherdovich@suse.cz>
Subject: [PATCH] x86, sched: Prevent divisions by zero in frequency invariant accounting
Date: Wed, 22 Apr 2020 16:40:55 +0200	[thread overview]
Message-ID: <20200422144055.18171-1-ggherdovich@suse.cz> (raw)

The product mcnt * arch_max_freq_ratio could be zero if it overflows u64.

For context, a large value for arch_max_freq_ratio would be 5000,
corresponding to a turbo_freq/base_freq ratio of 5 (normally it's more like
1500-2000). A large increment frequency for the MPERF counter would be 5GHz
(the base clock of all CPUs on the market today is less than that). With
these figures, a CPU would need to go without a scheduler tick for around 8
days for the u64 overflow to happen. It is unlikely, but the check is
warranted.

Signed-off-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
---
 arch/x86/kernel/smpboot.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 8c89e4d9ad28..fb71395cbcad 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -2055,14 +2055,14 @@ void arch_scale_freq_tick(void)
 
 	acnt = aperf - this_cpu_read(arch_prev_aperf);
 	mcnt = mperf - this_cpu_read(arch_prev_mperf);
-	if (!mcnt)
-		return;
 
 	this_cpu_write(arch_prev_aperf, aperf);
 	this_cpu_write(arch_prev_mperf, mperf);
 
 	acnt <<= 2*SCHED_CAPACITY_SHIFT;
 	mcnt *= arch_max_freq_ratio;
+	if (!mcnt)
+		return;
 
 	freq_scale = div64_u64(acnt, mcnt);
 
-- 
2.16.4

