From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-18.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_IN_DEF_DKIM_WL autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 16464C432BE for ; Wed, 4 Aug 2021 22:19:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E511660F35 for ; Wed, 4 Aug 2021 22:19:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235257AbhHDWT5 (ORCPT ); Wed, 4 Aug 2021 18:19:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49546 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235175AbhHDWT4 (ORCPT ); Wed, 4 Aug 2021 18:19:56 -0400 Received: from mail-ot1-x32e.google.com (mail-ot1-x32e.google.com [IPv6:2607:f8b0:4864:20::32e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 53950C061798 for ; Wed, 4 Aug 2021 15:19:43 -0700 (PDT) Received: by mail-ot1-x32e.google.com with SMTP id v9-20020a9d60490000b02904f06fc590dbso3134206otj.4 for ; Wed, 04 Aug 2021 15:19:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=fIeDGb81HEa/IJ5OXkRbKetCo9gTIt6kKZjhA2DPOzc=; b=M8ipkm04J9ZGePM4PMk62q3rG4n6ksWm61P98tEQF81wH9EEavJbqwECZ5uEzr4Q27 /w0ohBflOWp5rFFxsBFYZJhdj8Zfs8oyTl1rVVfP1ulP5J1OxcSFcbqVRUJ3CkGbXa81 +bwl73Zcr8ByhEiE1ws7/8ebdlmUKCJ1+zkQH/kDL3MKeuoyszmspyHAljmZuL8xSrKB DdZNU6o+pMeLUgRwFCv/R6zYHVBF9UFwxfW6gQ4DD8WVkxoE6jTp1hrRAAfl28Ypi9ko zwfomSxHWKmmT34MnujIrqAL4hRjlhkcRD0nifc8NdOrrPvqNu71S8y3ClRaAmxRszBC Y5hA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=fIeDGb81HEa/IJ5OXkRbKetCo9gTIt6kKZjhA2DPOzc=; b=sXgp1WAmWACvey6WgwQnnvSXFGvvsYNmWPOBWwKJM5E1qVnBt/1CACoD5AsgRCSIeU kx+NqkBQfge1kfzP31cYH5HSjJmv+vdNt8ixqjnFjm8N77WuIKpo+r59BaNHuPa+zWeD 1ArRQ41w2DWERJA4qG1+td0yYK7Md7UZsjdSRkhmLlr2FcfJneeIRKcmeUkqElbzDuF1 UD2wdQRinF+7cYMlQm7XLIg13YISZbeptk7CC3HqpNQWpjOnocAmKq6f6z/IClAOSj9A uMVyZVBYFhX24JR5qOZtmDiGcAviWF6kFvSFYvww9wnjJApaLjOUdex0AngBzWy31x/l nMvQ== X-Gm-Message-State: AOAM530jfcfEEgfNsTW530vpLeFhS4SffPiJLFIbyJDDAlgWz7C6X7n+ x/KplCMfTBFXWVNmN2guwK6amGdPy3mlQXtDvtSmuQ== X-Google-Smtp-Source: ABdhPJxiWFtLtbQZDHzmp15IKxv8wOz2ClzTL6LLruJYPn66EMJRznjCSucbOVPpnEq+m9gMByWN53k3qOuogdTVC98= X-Received: by 2002:a9d:63c6:: with SMTP id e6mr1313817otl.295.1628115582214; Wed, 04 Aug 2021 15:19:42 -0700 (PDT) MIME-Version: 1.0 References: <20210804214609.1096003-1-seanjc@google.com> In-Reply-To: <20210804214609.1096003-1-seanjc@google.com> From: Jim Mattson Date: Wed, 4 Aug 2021 15:19:30 -0700 Message-ID: Subject: Re: [PATCH] KVM: x86/mmu: Fix per-cpu counter corruption on 32-bit builds To: Sean Christopherson Cc: Paolo Bonzini , Vitaly Kuznetsov , Wanpeng Li , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ben Gardon Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 4, 2021 at 2:46 PM Sean Christopherson wrote: > > Take a signed 'long' instead of an 'unsigned long' for the number of > pages to add/subtract to the total number of pages used by the MMU. This > fixes a zero-extension bug on 32-bit kernels that effectively corrupts > the per-cpu counter used by the shrinker. > > Per-cpu counters take a signed 64-bit value on both 32-bit and 64-bit > kernels, whereas kvm_mod_used_mmu_pages() takes an unsigned long and thus > an unsigned 32-bit value on 32-bit kernels. As a result, the value used > to adjust the per-cpu counter is zero-extended (unsigned -> signed), not > sign-extended (signed -> signed), and so KVM's intended -1 gets morphed to > 4294967295 and effectively corrupts the counter. > > This was found by a staggering amount of sheer dumb luck when running > kvm-unit-tests on a 32-bit KVM build. The shrinker just happened to kick > in while running tests and do_shrink_slab() logged an error about trying > to free a negative number of objects. The truly lucky part is that the > kernel just happened to be a slightly stale build, as the shrinker no > longer yells about negative objects as of commit 18bb473e5031 ("mm: > vmscan: shrink deferred objects proportional to priority"). > > vmscan: shrink_slab: mmu_shrink_scan+0x0/0x210 [kvm] negative objects to delete nr=-858993460 > > Fixes: bc8a3d8925a8 ("kvm: mmu: Fix overflow on kvm mmu page limit calculation") > Cc: stable@vger.kernel.org > Cc: Ben Gardon > Signed-off-by: Sean Christopherson Ouch! Reviewed-by: Jim Mattson