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=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 D413EC48BE0 for ; Fri, 11 Jun 2021 07:27:06 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9D2A6613AE for ; Fri, 11 Jun 2021 07:27:06 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9D2A6613AE Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ffwll.ch Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 0E9AF6EE3A; Fri, 11 Jun 2021 07:27:06 +0000 (UTC) Received: from mail-ot1-x32e.google.com (mail-ot1-x32e.google.com [IPv6:2607:f8b0:4864:20::32e]) by gabe.freedesktop.org (Postfix) with ESMTPS id A8C876EE3A for ; Fri, 11 Jun 2021 07:27:04 +0000 (UTC) Received: by mail-ot1-x32e.google.com with SMTP id l15-20020a05683016cfb02903fca0eacd15so2281798otr.7 for ; Fri, 11 Jun 2021 00:27:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=ef52vRx74fc5UCfQHCig8+5esV/3pCK5+NiUbiLHnvQ=; b=Y8ThRrG17JeXYR82682Is0eVQuuGT56zmi/UILuLbklbnOVQnRJ1XIFjtxrdreVOaF GKaqSSnp7EA3r+zs6//WcDeAM/4EyuYJhcPtU5c8VEUluGgfIZNKeJJQBqWrcKIRyRXr OM0MezCo/yFyiRRiG1Zkhf1jxbtS+PdNwVLn8= 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=ef52vRx74fc5UCfQHCig8+5esV/3pCK5+NiUbiLHnvQ=; b=eHQDVwrPKc21QIdj0p4uyHtzDIii2293LtEumhlPi2g5oKEqGfd1bD981EXmpjxeqt SDneWlz6uQCITzLSH6z95HPW//Bacp6kIw9JKep65DnXTEgdzU65aJS0GIu8Ye7CW7Q4 RJaAbUTTYg8umfa4h1JVLglr6kOPR4d9lzKDg1g+VuKnHHAmpe/lx90/1S17+LajgFlP CfrkTWrWes9L0FTHCkLMhNNs1CjG8SvyiSkPqvG7sfwvzRG2ju+uyzkE9QhEyIwD8ypp 57xRGtvgyuNSqCeL9bzZPsojnyNAsrwZS7kxX5lSQ+K2tzLaLLpWw447bILHaFfkiBPV kfpQ== X-Gm-Message-State: AOAM530N5Kf3+J/k8pvXwUh6fRuvt8S4Wi4q1C1gkK4MY06DE2rEz5RL EP8oTVLlufusuPzWdqWmqAt7SQDAIURpjGv9TjTt2Q== X-Google-Smtp-Source: ABdhPJz3IOQqQF+lRvx53nYLgYJH0he5j+6eXuQM7b70P1oLbyxSfT5qUOpvpqgUteIugcKijfjghWgWanPrcPLy+6E= X-Received: by 2002:a9d:27a4:: with SMTP id c33mr1933399otb.281.1623396423931; Fri, 11 Jun 2021 00:27:03 -0700 (PDT) MIME-Version: 1.0 References: <20210609092119.173590-1-desmondcheongzx@gmail.com> In-Reply-To: From: Daniel Vetter Date: Fri, 11 Jun 2021 09:26:52 +0200 Message-ID: Subject: Re: [PATCH] drm: Lock pointer access in drm_master_release() To: Desmond Cheong Zhi Xi Content-Type: text/plain; charset="UTF-8" X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Dave Airlie , Greg KH , Linux Kernel Mailing List , dri-devel , Thomas Zimmermann , Shuah Khan , linux-kernel-mentees@lists.linuxfoundation.org, Dan Carpenter Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Fri, Jun 11, 2021 at 4:18 AM Desmond Cheong Zhi Xi wrote: On 11/6/21 12:48 am, Daniel Vetter wrote: > > On Thu, Jun 10, 2021 at 11:21:39PM +0800, Desmond Cheong Zhi Xi wrote: > >> On 10/6/21 6:10 pm, Daniel Vetter wrote: > >>> On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote: > >>>> This patch eliminates the following smatch warning: > >>>> drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex' > >>>> > >>>> The 'file_priv->master' field should be protected by the mutex lock to > >>>> '&dev->master_mutex'. This is because other processes can concurrently > >>>> modify this field and free the current 'file_priv->master' > >>>> pointer. This could result in a use-after-free error when 'master' is > >>>> dereferenced in subsequent function calls to > >>>> 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'. > >>>> > >>>> An example of a scenario that would produce this error can be seen > >>>> from a similar bug in 'drm_getunique()' that was reported by Syzbot: > >>>> https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803 > >>>> > >>>> In the Syzbot report, another process concurrently acquired the > >>>> device's master mutex in 'drm_setmaster_ioctl()', then overwrote > >>>> 'fpriv->master' in 'drm_new_set_master()'. The old value of > >>>> 'fpriv->master' was subsequently freed before the mutex was unlocked. > >>>> > >>>> Reported-by: Dan Carpenter > >>>> Signed-off-by: Desmond Cheong Zhi Xi > >>> > >>> Thanks a lot. I've done an audit of this code, and I found another > >>> potential problem in drm_is_current_master. The callers from drm_auth.c > >>> hold the dev->master_mutex, but all the external ones dont. I think we > >>> need to split this into a _locked function for use within drm_auth.c, and > >>> the exported one needs to grab the dev->master_mutex while it's checking > >>> master status. Ofc there will still be races, those are ok, but right now > >>> we run the risk of use-after free problems in drm_lease_owner. > >>> > >>> Are you up to do that fix too? > >>> > >> > >> Hi Daniel, > >> > >> Thanks for the pointer, I'm definitely up for it! > >> > >>> I think the drm_lease.c code also needs an audit, there we'd need to make > >>> sure that we hold hold either the lock or a full master reference to avoid > >>> the use-after-free issues here. > >>> > >> > >> I'd be happy to look into drm_lease.c as well. > >> > >>> Patch merged to drm-misc-fixes with cc: stable. > >>> -Daniel > >>> > >>>> --- > >>>> drivers/gpu/drm/drm_auth.c | 3 ++- > >>>> 1 file changed, 2 insertions(+), 1 deletion(-) > >>>> > >>>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c > >>>> index f00e5abdbbf4..b59b26a71ad5 100644 > >>>> --- a/drivers/gpu/drm/drm_auth.c > >>>> +++ b/drivers/gpu/drm/drm_auth.c > >>>> @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv) > >>>> void drm_master_release(struct drm_file *file_priv) > >>>> { > >>>> struct drm_device *dev = file_priv->minor->dev; > >>>> - struct drm_master *master = file_priv->master; > >>>> + struct drm_master *master; > >>>> > >>>> mutex_lock(&dev->master_mutex); > >>>> + master = file_priv->master; > >>>> if (file_priv->magic) > >>>> idr_remove(&file_priv->master->magic_map, file_priv->magic); > >>>> -- > >>>> 2.25.1 > >>>> > >>> > >> > >> From what I can see, there are other places in the kernel that could use the > >> _locked version of drm_is_current_master as well, such as drm_mode_getfb in > >> drm_framebuffer.c. I'll take a closer look, and if the changes make sense > >> I'll prepare a patch series for them. > > > > Oh maybe we have a naming confusion: the _locked is the one where the > > caller must grab the lock already, whereas drm_is_current_master would > > grab the master_mutex internally to do the check. The one in > > drm_framebuffer.c looks like it'd need the internal one since there's no > > other need to grab the master_mutex. > > -Daniel > > > > Ah ok got it, I think I confused myself earlier. > > Just to check, may I include you in a Reported-by: tag? Sure. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch