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[23.128.96.18]) by mx.google.com with ESMTP id x12si10674539iom.57.2021.07.21.11.20.56; Wed, 21 Jul 2021 11:21:23 -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=@ffwll.ch header.s=google header.b=QOigZ7HW; 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 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237722AbhGUMne (ORCPT + 99 others); Wed, 21 Jul 2021 08:43:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40498 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237711AbhGUMn2 (ORCPT ); Wed, 21 Jul 2021 08:43:28 -0400 Received: from mail-oi1-x234.google.com (mail-oi1-x234.google.com [IPv6:2607:f8b0:4864:20::234]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 95352C061762 for ; Wed, 21 Jul 2021 06:24:04 -0700 (PDT) Received: by mail-oi1-x234.google.com with SMTP id y66so2815948oie.7 for ; Wed, 21 Jul 2021 06:24:04 -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:content-transfer-encoding; bh=J7kjvLLVDgJb9U07ZLlILvOE2QjtW/z+1WytRCj7dp8=; b=QOigZ7HWMPOvG1xoIIHD8wdnyLBZ4mo25yN8IHWf0/d8F9p0Zv2UidhAn8lVEpR/yL 7ZTwOA9PZn/GSdyaJLZmXGBguYqrUZQrUmnWDyuiICONq29uw2rrqDG7XOG8QFr5m0S8 3f1zb5LX2McJ0CPcKT68UqknK6lPLYNygpRVA= 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:content-transfer-encoding; bh=J7kjvLLVDgJb9U07ZLlILvOE2QjtW/z+1WytRCj7dp8=; b=d24hrOkQdoKAFO/O4LHdpgtYjzrX0wmJZBYntv/rhxdlwhOtGmd6GTGE9JrQuN4mXB QE+9CT0TloqN1ltH54KrSgxJ6c9J8h/q8F18Bwq1oYbJ+tb8eccu9LZTCQK89HBxmLTi iHcB6huRvEhmFt5VDit69o61IBFTyWEV3/T1ZQvIY3Z4SozqfM6GVun0/MWeqbqjg13L k+Jq/V+a2yGLwRp6sn6j/vnaD7TOnzu/czJDfvxXOn/ZFBsPujDjqWujLzWv5yRDUMDj vuCb20ixayWAb+jhcyhai8f9ykAH3uUJeiexalPerE16qvecHMO/tkSV7n94ZDiowVKE oU2w== X-Gm-Message-State: AOAM531AblRAxlcafyExKfuoM1t0poLNMKsOsWUVLblFhcvS+nrGnnEv YQdWiYyqiN933Ru5orQy2xYDqVAAwc4npn4VMeWfeA== X-Received: by 2002:aca:d4cf:: with SMTP id l198mr2546927oig.14.1626873843909; Wed, 21 Jul 2021 06:24:03 -0700 (PDT) MIME-Version: 1.0 References: <20210712043508.11584-1-desmondcheongzx@gmail.com> <50c5582b-c674-4ef8-585f-7a3d78a49f85@gmail.com> <52c4207a-6830-01c9-a28c-635c68de3e14@gmail.com> In-Reply-To: <52c4207a-6830-01c9-a28c-635c68de3e14@gmail.com> From: Daniel Vetter Date: Wed, 21 Jul 2021 15:23:52 +0200 Message-ID: Subject: Re: [PATCH v8 0/5] drm: address potential UAF bugs with drm_master ptrs To: Desmond Cheong Zhi Xi Cc: Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Dave Airlie , Sumit Semwal , =?UTF-8?Q?Christian_K=C3=B6nig?= , dri-devel , intel-gfx , Linux Kernel Mailing List , "open list:DMA BUFFER SHARING FRAMEWORK" , "moderated list:DMA BUFFER SHARING FRAMEWORK" , Shuah Khan , Greg KH , linux-kernel-mentees@lists.linuxfoundation.org, Emil Velikov Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 21, 2021 at 2:44 PM Desmond Cheong Zhi Xi wrote: > On 21/7/21 6:29 pm, Daniel Vetter wrote: > > On Wed, Jul 21, 2021 at 6:12 AM Desmond Cheong Zhi Xi > > wrote: > >> On 21/7/21 2:24 am, Daniel Vetter wrote: > >>> On Mon, Jul 12, 2021 at 12:35:03PM +0800, Desmond Cheong Zhi Xi wrote= : > >>>> Hi, > >>>> > >>>> In the previous thread on this series we decided to remove a patch t= hat was violating a lockdep requirement in drm_lease. In addition to this c= hange, I took a closer look at the CI logs for the Basic Acceptance Tests a= nd noticed that another regression was introduced. The new patch 2 is a res= ponse to this. > >>>> > >>>> Overall, this series addresses potential use-after-free errors when = dereferencing pointers to struct drm_master. These were identified after on= e such bug was caught by Syzbot in drm_getunique(): > >>>> https://syzkaller.appspot.com/bug?id=3D148d2f1dfac64af52ffd27b661981= a540724f803 > >>>> > >>>> The series is broken up into five patches: > >>>> > >>>> 1. Move a call to drm_is_current_master() out from a section locked = by &dev->mode_config.mutex in drm_mode_getconnector(). This patch does not = apply to stable. > >>>> > >>>> 2. Move a call to drm_is_current_master() out from the RCU read-side= critical section in drm_clients_info(). > >>>> > >>>> 3. Implement a locked version of drm_is_current_master() function th= at's used within drm_auth.c. > >>>> > >>>> 4. Serialize drm_file.master by introducing a new spinlock that's he= ld whenever the value of drm_file.master changes. > >>>> > >>>> 5. Identify areas in drm_lease.c where pointers to struct drm_master= are dereferenced, and ensure that the master pointers are not freed during= use. > >>>> > >>>> v7 -> v8: > >>>> - Remove the patch that moves the call to _drm_lease_held out from t= he section locked by &dev->mode_config.idr_mutex in __drm_mode_object_find.= This patch violated an existing lockdep requirement as reported by the int= el-gfx CI. > >>>> - Added a new patch that moves a call to drm_is_current_master out f= rom the RCU critical section in drm_clients_info. This was reported by the = intel-gfx CI. > >>>> > >>>> v6 -> v7: > >>>> - Modify code alignment as suggested by the intel-gfx CI. > >>>> - Add a new patch to the series that adds a new lock to serialize dr= m_file.master, in response to the lockdep splat by the intel-gfx CI. > >>>> - Update drm_file_get_master to use the new drm_file.master_lock ins= tead of drm_device.master_mutex, in response to the lockdep splat by the in= tel-gfx CI. > >>>> > >>>> v5 -> v6: > >>>> - Add a new patch to the series that moves the call to _drm_lease_he= ld out from the section locked by &dev->mode_config.idr_mutex in __drm_mode= _object_find. > >>>> - Clarify the kerneldoc for dereferencing drm_file.master, as sugges= ted by Daniel Vetter. > >>>> - Refactor error paths with goto labels so that each function only h= as a single drm_master_put(), as suggested by Emil Velikov. > >>>> - Modify comparisons to NULL into "!master", as suggested by the int= el-gfx CI. > >>>> > >>>> v4 -> v5: > >>>> - Add a new patch to the series that moves the call to drm_is_curren= t_master in drm_mode_getconnector out from the section locked by &dev->mode= _config.mutex. > >>>> - Additionally, added a missing semicolon to the patch, caught by th= e intel-gfx CI. > >>>> > >>>> v3 -> v4: > >>>> - Move the call to drm_is_current_master in drm_mode_getconnector ou= t from the section locked by &dev->mode_config.mutex. As suggested by Danie= l Vetter. This avoids a circular lock lock dependency as reported here http= s://patchwork.freedesktop.org/patch/440406/ > >>>> - Inside drm_is_current_master, instead of grabbing &fpriv->master->= dev->master_mutex, we grab &fpriv->minor->dev->master_mutex to avoid derefe= rencing a null ptr if fpriv->master is not set. > >>>> - Modify kerneldoc formatting for drm_file.master, as suggested by D= aniel Vetter. > >>>> - Additionally, add a file_priv->master NULL check inside drm_file_g= et_master, and handle the NULL result accordingly in drm_lease.c. As sugges= ted by Daniel Vetter. > >>>> > >>>> v2 -> v3: > >>>> - Move the definition of drm_is_current_master and the _locked versi= on higher up in drm_auth.c to avoid needing a forward declaration of drm_is= _current_master_locked. As suggested by Daniel Vetter. > >>>> - Instead of leaking drm_device.master_mutex into drm_lease.c to pro= tect drm_master pointers, add a new drm_file_get_master() function that ret= urns drm_file->master while increasing its reference count, to prevent drm_= file->master from being freed. As suggested by Daniel Vetter. > >>>> > >>>> v1 -> v2: > >>>> - Move the lock and assignment before the DRM_DEBUG_LEASE in drm_mod= e_get_lease_ioctl, as suggested by Emil Velikov. > >>> > >>> Apologies for the delay, I missed your series. Maybe just ping next t= ime > >>> around there's silence. > >>> > >>> Looks all great, merged to drm-misc-next. Given how complex this was = I'm > >>> vary of just pushing this to -fixes without some solid testing. > >>> > >> > >> Hi Daniel, > >> > >> Thanks for merging, more testing definitely sounds good to me. > >> > >>> One thing I noticed is that drm_is_current_master could just use the > >>> spinlock, since it's only doing a read access. Care to type up that p= atch? > >>> > >> > >> I thought about this too, but I'm not sure if that's the best solution= . > >> > >> drm_is_current_master calls drm_lease_owner which then walks up the tr= ee > >> of master lessors. The spinlock protects the master of the current drm > >> file, but subsequent lessors aren't protected without holding the > >> device's master mutex. > > > > But this isn't a fpriv->master pointer, but a master->lessor pointer. > > Which should never ever be able to change (we'd have tons of uaf bugs > > around drm_lease_owner otherwise). So I don't think there's anything > > that dev->master_lock protects here that fpriv->master_lookup_lock > > doesn't protect already? > > > > Or am I missing something? > > > The comment in the struct drm_master says it's protected by > > mode_config.idr_mutex, but that only applies to the idrs and lists I > > think. > > > > Ah you're right, I also completely forgot that lessees hold a reference > to their lessor so nothing will be freed as long as the spinlock is > held. I'll prepare that patch then, thanks for pointing it out. btw since we now looked at all this in detail, can you perhaps do a patch to update the kerneldoc for all the lease fields in struct drm_master? I think moving them to the inline style and then adding comments for each field how locking/lifetime rules work would be really good. Since right now it's all fresh from for us. -Daniel > >>> Also, do you plan to look into that idea we've discussed to flush pen= ding > >>> access when we revoke a master or a lease? I think that would be real= ly > >>> nice improvement here. > >>> -Daniel > >>> > >> > >> Yup, now that the potential UAFs are addressed (hopefully), I'll take = a > >> closer look and propose a patch for this. > > > > Thanks a lot. > > -Daniel > > > >> > >> Best wishes, > >> Desmond > >> > >>>> > >>>> Desmond Cheong Zhi Xi (5): > >>>> drm: avoid circular locks in drm_mode_getconnector > >>>> drm: avoid blocking in drm_clients_info's rcu section > >>>> drm: add a locked version of drm_is_current_master > >>>> drm: serialize drm_file.master with a new spinlock > >>>> drm: protect drm_master pointers in drm_lease.c > >>>> > >>>> drivers/gpu/drm/drm_auth.c | 93 ++++++++++++++++++++++++----= ----- > >>>> drivers/gpu/drm/drm_connector.c | 5 +- > >>>> drivers/gpu/drm/drm_debugfs.c | 3 +- > >>>> drivers/gpu/drm/drm_file.c | 1 + > >>>> drivers/gpu/drm/drm_lease.c | 81 +++++++++++++++++++++------- > >>>> include/drm/drm_auth.h | 1 + > >>>> include/drm/drm_file.h | 18 +++++-- > >>>> 7 files changed, 152 insertions(+), 50 deletions(-) > >>>> > >>>> -- > >>>> 2.25.1 > >>>> > >>> > >> > > > > > --=20 Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch