From patchwork Thu May 30 20:34:58 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kamal Mostafa X-Patchwork-Id: 247740 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from huckleberry.canonical.com (huckleberry.canonical.com [91.189.94.19]) by ozlabs.org (Postfix) with ESMTP id E348C2C009C for ; Fri, 31 May 2013 06:37:22 +1000 (EST) Received: from localhost ([127.0.0.1] helo=huckleberry.canonical.com) by huckleberry.canonical.com with esmtp (Exim 4.76) (envelope-from ) id 1Ui9b3-0004Ak-FF; Thu, 30 May 2013 20:37:17 +0000 Received: from youngberry.canonical.com ([91.189.89.112]) by huckleberry.canonical.com with esmtp (Exim 4.76) (envelope-from ) id 1Ui9Yr-0002vz-Iz for kernel-team@lists.ubuntu.com; Thu, 30 May 2013 20:35:01 +0000 Received: from c-67-160-231-42.hsd1.ca.comcast.net ([67.160.231.42] helo=fourier) by youngberry.canonical.com with esmtpsa (TLS1.0:DHE_RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1Ui9Yq-0002lR-Lg; Thu, 30 May 2013 20:35:00 +0000 Received: from kamal by fourier with local (Exim 4.80) (envelope-from ) id 1Ui9Yo-0003TT-BA; Thu, 30 May 2013 13:34:58 -0700 From: Kamal Mostafa To: Jiri Kosina Subject: [ 3.8.y.z extended stable ] Patch "random: fix accounting race condition with lockless irq entropy_count" has been added to staging queue Date: Thu, 30 May 2013 13:34:58 -0700 Message-Id: <1369946098-13321-1-git-send-email-kamal@canonical.com> X-Mailer: git-send-email 1.8.1.2 X-Extended-Stable: 3.8 Cc: Theodore Ts'o , Greg KH , Kamal Mostafa , kernel-team@lists.ubuntu.com, Andrew Morton , Linus Torvalds X-BeenThere: kernel-team@lists.ubuntu.com X-Mailman-Version: 2.1.14 Precedence: list List-Id: Kernel team discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , MIME-Version: 1.0 Errors-To: kernel-team-bounces@lists.ubuntu.com Sender: kernel-team-bounces@lists.ubuntu.com This is a note to let you know that I have just added a patch titled random: fix accounting race condition with lockless irq entropy_count to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree which can be found at: http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue This patch is scheduled to be released in version 3.8.13.2. If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.8.y.z tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Kamal ------ From f0d568d95f929d59bc91b5d3c38beddda81abd51 Mon Sep 17 00:00:00 2001 From: Jiri Kosina Date: Fri, 24 May 2013 15:55:33 -0700 Subject: random: fix accounting race condition with lockless irq entropy_count update commit 10b3a32d292c21ea5b3ad5ca5975e88bb20b8d68 upstream. Commit 902c098a3663 ("random: use lockless techniques in the interrupt path") turned IRQ path from being spinlock protected into lockless cmpxchg-retry update. That commit removed r->lock serialization between crediting entropy bits from IRQ context and accounting when extracting entropy on userspace read path, but didn't turn the r->entropy_count reads/updates in account() to use cmpxchg as well. It has been observed, that under certain circumstances this leads to read() on /dev/urandom to return 0 (EOF), as r->entropy_count gets corrupted and becomes negative, which in turn results in propagating 0 all the way from account() to the actual read() call. Convert the accounting code to be the proper lockless counterpart of what has been partially done by 902c098a3663. Signed-off-by: Jiri Kosina Cc: Theodore Ts'o Cc: Greg KH Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Kamal Mostafa --- drivers/char/random.c | 24 ++++++++++++++++-------- 1 file changed, 16 insertions(+), 8 deletions(-) -- 1.8.1.2 diff --git a/drivers/char/random.c b/drivers/char/random.c index ae872f8..b7dc45c 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -865,16 +865,24 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min, if (r->entropy_count / 8 < min + reserved) { nbytes = 0; } else { + int entropy_count, orig; +retry: + entropy_count = orig = ACCESS_ONCE(r->entropy_count); /* If limited, never pull more than available */ - if (r->limit && nbytes + reserved >= r->entropy_count / 8) - nbytes = r->entropy_count/8 - reserved; - - if (r->entropy_count / 8 >= nbytes + reserved) - r->entropy_count -= nbytes*8; - else - r->entropy_count = reserved; + if (r->limit && nbytes + reserved >= entropy_count / 8) + nbytes = entropy_count/8 - reserved; + + if (entropy_count / 8 >= nbytes + reserved) { + entropy_count -= nbytes*8; + if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig) + goto retry; + } else { + entropy_count = reserved; + if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig) + goto retry; + } - if (r->entropy_count < random_write_wakeup_thresh) + if (entropy_count < random_write_wakeup_thresh) wakeup_write = 1; }