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[23.128.96.18]) by mx.google.com with ESMTP id t8si3542726edr.491.2021.06.10.19.20.24; Thu, 10 Jun 2021 19:21:08 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=aPH2Rj6G; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231284AbhFKCUk (ORCPT + 99 others); Thu, 10 Jun 2021 22:20:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33190 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230281AbhFKCUk (ORCPT ); Thu, 10 Jun 2021 22:20:40 -0400 Received: from mail-pg1-x535.google.com (mail-pg1-x535.google.com [IPv6:2607:f8b0:4864:20::535]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1D484C061574 for ; Thu, 10 Jun 2021 19:18:28 -0700 (PDT) Received: by mail-pg1-x535.google.com with SMTP id l1so1257535pgm.1 for ; Thu, 10 Jun 2021 19:18:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:content-language:content-transfer-encoding; bh=CxgTlwmogoHhOULSFq3GJiDbsRi88PXUgGnGmm64QIk=; b=aPH2Rj6GwUHE43OWEfQPUaa1wYw6SyzIomh4HiyRwux/MIJA/vVXxze7aXwz3HUt+H 9pB7OWEbdwAoXrga89jq2Y4jSIdaGgx1FyMjJ7F85COtVOd553YmR2DHyaTmAyLRAvIb A+XLas/CxN6Yw/j7uCYPmHxOJ148N18bS1sgfV8gN8zOCxH6ZX2uFFj1iCpZGFbiqiR1 IKkh3chxuGJg5fN4jNohvhjFajK02UxfrBPG28fSwEoJKbWdi2kzyi8Lyqx/bxWKqsIQ rJJqszvCD6WZMLR/RRnuAdTFiGF47FwWxeWjLru9Kg+hhIbwqtioVe9yFxKavSOgnvyu RyPw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=CxgTlwmogoHhOULSFq3GJiDbsRi88PXUgGnGmm64QIk=; b=aRJFBDztqE9z8g/mvS7Sk4czfk2AFMzCTLHfpL1Gz+Tky7kuKAQ30aTGSFC/82Iq9P 7tsHLuzSHIT8D/ot6xP6+gghG7pvMetyrJ3dHaxqJun+nc6H4Dqr8U5Rwl/DXy5YZYGW zCrUamlw/g+fo7lvfnIOQ0naoLRdLp42j1QcLZvqrae2nnliTxzpqDrItEiD+87054A1 qeI91iAPw6HYlU3aLjv3QaPXP/qCUtMp1W0DUjmrCBaClI6Ciyulv9GuVNedTnczZwmb If9HGNi4OUsHHJOPctGaR0ekZIzo0B5FJNlHBvrK6pGwQPlLI0BmQ9fCk2n1YtwTjHjH AMug== X-Gm-Message-State: AOAM53177AuhPvb+dj+LWZvADkUd5mvaXdmcFs9/BLtQ8sqhLXRJI1Bh ba3v2QvIXA2c2w/lnnCcMjM= X-Received: by 2002:a62:bd14:0:b029:2de:8bf7:2df8 with SMTP id a20-20020a62bd140000b02902de8bf72df8mr5856590pff.60.1623377907516; Thu, 10 Jun 2021 19:18:27 -0700 (PDT) Received: from [192.168.1.237] ([118.200.190.93]) by smtp.gmail.com with ESMTPSA id o16sm3439636pfu.75.2021.06.10.19.18.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Jun 2021 19:18:27 -0700 (PDT) Subject: Re: [PATCH] drm: Lock pointer access in drm_master_release() To: maarten.lankhorst@linux.intel.com, mripard@kernel.org, tzimmermann@suse.de, airlied@linux.ie, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, skhan@linuxfoundation.org, gregkh@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org, Dan Carpenter References: <20210609092119.173590-1-desmondcheongzx@gmail.com> From: Desmond Cheong Zhi Xi Message-ID: Date: Fri, 11 Jun 2021 10:18:22 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?