From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f195.google.com ([209.85.223.195]:54612 "EHLO mail-io0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755829AbdKGWQV (ORCPT ); Tue, 7 Nov 2017 17:16:21 -0500 From: Eric Biggers To: linux-crypto@vger.kernel.org, Herbert Xu Cc: Tudor-Dan Ambarus , Mat Martineau , Salvatore Benedetto , keyrings@vger.kernel.org, linux-kernel@vger.kernel.org, Eric Biggers , stable@vger.kernel.org Subject: [PATCH v2] lib/mpi: call cond_resched() from mpi_powm() loop Date: Tue, 7 Nov 2017 14:15:27 -0800 Message-Id: <20171107221527.98433-1-ebiggers3@gmail.com> Sender: stable-owner@vger.kernel.org List-ID: From: Eric Biggers On a non-preemptible kernel, if KEYCTL_DH_COMPUTE is called with the largest permitted inputs (16384 bits), the kernel spends 10+ seconds doing modular exponentiation in mpi_powm() without rescheduling. If all threads do it, it locks up the system. Moreover, it can cause rcu_sched-stall warnings. Notwithstanding the insanity of doing this calculation in kernel mode rather than in userspace, fix it by calling cond_resched() as each bit from the exponent is processed. It's still noninterruptible, but at least it's preemptible now. Do the cond_resched() once per bit rather than once per MPI limb because each limb might still easily take 100+ milliseconds on slow CPUs. Cc: # v4.12+ Signed-off-by: Eric Biggers --- lib/mpi/mpi-pow.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c index e24388a863a7..468fb7cd1221 100644 --- a/lib/mpi/mpi-pow.c +++ b/lib/mpi/mpi-pow.c @@ -26,6 +26,7 @@ * however I decided to publish this code under the plain GPL. */ +#include #include #include "mpi-internal.h" #include "longlong.h" @@ -256,6 +257,7 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) } e <<= 1; c--; + cond_resched(); } i--; -- 2.15.0.403.gc27cc4dac6-goog