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Fri, 11 Sep 2020 21:34:07 +0000 Date: Fri, 11 Sep 2020 14:34:02 -0700 From: Roman Gushchin To: Shakeel Butt CC: Andrew Morton , Linux MM , Johannes Weiner , Michal Hocko , Kernel Team , LKML , Vlastimil Babka , Subject: Re: [PATCH] mm: memcg/slab: fix racy access to page->mem_cgroup in mem_cgroup_from_obj() Message-ID: <20200911213402.GB1163084@carbon.dhcp.thefacebook.com> References: <20200910022435.2773735-1-guro@fb.com> <20200910224309.GB1307870@carbon.dhcp.thefacebook.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MWHPR1601CA0001.namprd16.prod.outlook.com (2603:10b6:300:da::11) To BYAPR15MB4136.namprd15.prod.outlook.com (2603:10b6:a03:96::24) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from carbon.dhcp.thefacebook.com (2620:10d:c090:400::5:44fc) by MWHPR1601CA0001.namprd16.prod.outlook.com (2603:10b6:300:da::11) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3370.16 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: 9b5b2726-fd00-42d5-6cb8-08d8569a6987 X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB4136.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Sep 2020 21:34:07.1508 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 5RUJufXjQF+U5h8VZxwCE1/G9xFC7jKCiwovvAKh4Jx02TcH4NAH58yQml9IdzP7 X-MS-Exchange-Transport-CrossTenantHeadersStamped: BY5PR15MB3652 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.687 definitions=2020-09-11_12:2020-09-10,2020-09-11 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 adultscore=0 suspectscore=0 bulkscore=0 priorityscore=1501 mlxscore=0 mlxlogscore=999 spamscore=0 malwarescore=0 phishscore=0 impostorscore=0 clxscore=1015 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009110175 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 11, 2020 at 09:21:49AM -0700, Shakeel Butt wrote: > On Thu, Sep 10, 2020 at 3:43 PM Roman Gushchin wrote: > > > > Forgot to cc stable@, an updated version is below. > > > > Thanks! > > > > -- > > > > From fe61af45ae570b143ca783ba4d013a0a2b923a15 Mon Sep 17 00:00:00 2001 > > From: Roman Gushchin > > Date: Wed, 9 Sep 2020 12:19:37 -0700 > > Subject: [PATCH] mm: memcg/slab: fix racy access to page->mem_cgroup in > > mem_cgroup_from_obj() > > > > mem_cgroup_from_obj() checks the lowest bit of the page->mem_cgroup > > pointer to determine if the page has an attached obj_cgroup vector > > instead of a regular memcg pointer. If it's not set, it simple returns > > the page->mem_cgroup value as a struct mem_cgroup pointer. > > > > The commit 10befea91b61 ("mm: memcg/slab: use a single set of > > kmem_caches for all allocations") changed the moment when this bit > > is set: if previously it was set on the allocation of the slab page, > > now it can be set well after, when the first accounted object is > > allocated on this page. > > > > It opened a race: if page->mem_cgroup is set concurrently after the > > first page_has_obj_cgroups(page) check, a pointer to the obj_cgroups > > array can be returned as a memory cgroup pointer. > > > > A simple check for page->mem_cgroup pointer for NULL before the > > page_has_obj_cgroups() check fixes the race. Indeed, if the pointer > > is not NULL, it's either a simple mem_cgroup pointer or a pointer > > to obj_cgroup vector. The pointer can be asynchronously changed > > from NULL to (obj_cgroup_vec | 0x1UL), but can't be changed > > from a valid memcg pointer to objcg vector or back. > > > > If the object passed to mem_cgroup_from_obj() is a slab object > > and page->mem_cgroup is NULL, it means that the object is not > > accounted, so the function must return NULL. > > > > I've discovered the race looking at the code, so far I haven't seen it > > in the wild. > > > > Fixes: 10befea91b61 ("mm: memcg/slab: use a single set of kmem_caches for all allocations") > > Signed-off-by: Roman Gushchin > > Cc: Johannes Weiner > > Cc: Vlastimil Babka > > Cc: Shakeel Butt > > Cc: stable@vger.kernel.org > > --- > > Is the caller list_lru_from_kmem() the concern or is this more about > making mem_cgroup_from_obj() more future proof? I was doing some refactorings around (see https://lore.kernel.org/linux-mm/20200910202659.1378404-1-guro@fb.com/T/#t ), and just noticed it from looking at the code. I'm not aware of any real life consequences at the moment. > > Also have you taken a look at [1]? I am still trying to figure out how > that is possible. > > [1] https://lore.kernel.org/lkml/20200901075321.GL4299@shao2-debian/ Hm, yeah, it's complicated. At the very first glance it looks like that the obj_cgroups vector is placed onto the very same page it describes, or at least it shares the kmem_cache with it, with some bad consequences. Could be something SLAB-specific, newer saw anything like that with SLUB. Or maybe it's completely unrelated and has been attributed to this commit by mistake. I've spent several hours running the provided test in a loop, but wasn't lucky enough to trigger it. Did you try? Thanks! Roman