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Wed, 19 Feb 2020 04:03:28 +0000 Subject: Re: possible deadlock in bpf_lru_push_free To: Hillf Danton CC: syzbot , , , , , , , , , References: <20200217052336.5556-1-hdanton@sina.com> <20200219021542.3304-1-hdanton@sina.com> From: Yonghong Song Message-ID: <7e8c1cf0-fccf-1ed9-40ed-3a13b2287cf8@fb.com> Date: Tue, 18 Feb 2020 20:03:06 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 In-Reply-To: <20200219021542.3304-1-hdanton@sina.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: CO1PR15CA0085.namprd15.prod.outlook.com (2603:10b6:101:20::29) To DM6PR15MB3001.namprd15.prod.outlook.com (2603:10b6:5:13c::16) MIME-Version: 1.0 Received: from marksan-mbp.DHCP.thefacebook.com (2620:10d:c090:400::5:982) by CO1PR15CA0085.namprd15.prod.outlook.com (2603:10b6:101:20::29) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2729.25 via Frontend Transport; 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X-Microsoft-Antispam-Message-Info: 17O4NBg10BM+SdzluzwwDZKGBGd9RLflxKr5CYX04RSOkftMDQATc7Fy9I/Pf4VzAiXsO4Uf2HDjOw0kDXg1Fu5JZYVYScyeaxk3xYj3LipISk/wQEmQq3410Tu8JVZGYtKZGE0FB4MaG7uPfyVli8TDnd8s8LjxwAbWEMyhG4kScT+MB3KFbuTsM3aRH3T9RYionXOd+MDFy/ogT39SM3RibiqrcqMeblrn35/Daxuo6Wu6Ts1grdTkkwhxKzMR+31CzFf/5ZY3jV5QeyNc2BFOzAk0jcNXIFu4Ps+48b4SUD3Bvtf01h8rH1P2ss96RJhhkXEB12wS8k/RoZ67/MfpW4xhb7fF9NXenLRIrBx8OQFWCgxUKp0aFe+r0GK383H1qOwXeTA2yWs+qdmYYAq4T+VtbLhvMesEEOk/bMkc6fDIQ/ABiHS2gRjt5BWX X-MS-Exchange-AntiSpam-MessageData: OcTdcRVNy3W4QyLXP5Xhydx2Jq7W8FKCcA/eKjHZGlzy8kD+LwP4225V/qKx4qJ6ICj5PoOFZL+s6Jxd8zFyY14BrOofvEpjjzJXGKLPsNBkHen/W+9MIFNo/sFxF5owwFj/5BsX6SOfr9qphkPzBnpCs99eKpIXowSL7VLufbmLmwaVFvcOcEKAZA+Ib0eG X-MS-Exchange-CrossTenant-Network-Message-Id: 9f46ffc7-35e5-4cd3-fd05-08d7b4f0acbb X-MS-Exchange-CrossTenant-OriginalArrivalTime: 19 Feb 2020 04:03:28.2014 (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: ncc4PglXqXvHac/KtZJwSdHBrvuGpoXPfeAIURjFdXGea66VtfcNzUW38fB9XqAf X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR15MB3912 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138,18.0.572 definitions=2020-02-18_08:2020-02-18,2020-02-18 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 mlxlogscore=717 mlxscore=0 suspectscore=2 lowpriorityscore=0 clxscore=1015 phishscore=0 adultscore=0 priorityscore=1501 malwarescore=0 bulkscore=0 impostorscore=0 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2002190028 X-FB-Internal: deliver Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 2/18/20 6:15 PM, Hillf Danton wrote: > > Hey > > On Tue, 18 Feb 2020 15:55:02 -0800 Yonghong Song wrote: >> >> Thanks for Martin for explanation! I think changing l->hash_node.next is >> unsafe here as another thread may execute on a different cpu and >> traverse the same list. It will see hash_node.next = NULL and it is > > Good catch. > >> unexpected. >> >> How about the following patch? >> > Looks nicer, thanks :P > >> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c >> index 2d182c4ee9d9..246ef0f2e985 100644 >> --- a/kernel/bpf/hashtab.c >> +++ b/kernel/bpf/hashtab.c >> @@ -56,6 +56,7 @@ struct htab_elem { >> union { >> struct bpf_htab *htab; >> struct pcpu_freelist_node fnode; >> + struct htab_elem *link; >> }; >> }; >> }; >> @@ -1256,6 +1257,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map, >> void __user *ukeys = u64_to_user_ptr(attr->batch.keys); >> void *ubatch = u64_to_user_ptr(attr->batch.in_batch); >> u32 batch, max_count, size, bucket_size; >> + struct htab_elem *node_to_free = NULL; >> u64 elem_map_flags, map_flags; >> struct hlist_nulls_head *head; >> struct hlist_nulls_node *n; >> @@ -1370,9 +1372,14 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map, >> } >> if (do_delete) { >> hlist_nulls_del_rcu(&l->hash_node); >> - if (is_lru_map) >> - bpf_lru_push_free(&htab->lru, &l->lru_node); >> - else >> + if (is_lru_map) { >> + /* l->hnode overlaps with *l->hash_node.pprev > > nit: looks like you mean l->link Yes, my previous attempt uses "hnode" and later changed to "link" but forget to change the comments. Will post a patch soon. > >> + * in memory. l->hash_node.pprev has been >> + * poisoned and nobody should access it. >> + */ >> + l->link = node_to_free; >> + node_to_free = l; >> + } else >> free_htab_elem(htab, l); >> } >> dst_key += key_size; >