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Fri, 6 Nov 2020 00:15:57 +0000 From: Song Liu To: Daniel Xu CC: "bpf@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii.nakryiko@gmail.com" , Kernel Team Subject: Re: [PATCH bpf v4 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator Thread-Topic: [PATCH bpf v4 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator Thread-Index: AQHWs9C5LVaWDjXRREKJL+Ztb1AwE6m6PF2A Date: Fri, 6 Nov 2020 00:15:57 +0000 Message-ID: <472C44CE-2DB2-48B1-8F58-0541F877AEA7@fb.com> References: <4ff12d0c19de63e7172d25922adfb83ae7c8691f.1604620776.git.dxu@dxuuu.xyz> In-Reply-To: <4ff12d0c19de63e7172d25922adfb83ae7c8691f.1604620776.git.dxu@dxuuu.xyz> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3608.120.23.2.4) authentication-results: dxuuu.xyz; dkim=none (message not signed) header.d=none;dxuuu.xyz; dmarc=none action=none header.from=fb.com; x-originating-ip: [2620:10d:c090:400::5:ca49] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 25cccf58-4a6b-4326-9a72-08d881e92289 x-ms-traffictypediagnostic: BYAPR15MB2647: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:1332; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB2999.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 25cccf58-4a6b-4326-9a72-08d881e92289 X-MS-Exchange-CrossTenant-originalarrivaltime: 06 Nov 2020 00:15:57.8522 (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: 9Lz2R8lWf8UO3o9IIhC5g2tUmspDFWXosin6vJ6X3G+Tih8gkl6Wrj1dRpxoWtQ5yhz6U9FDIiWoX+kwRguf8Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2647 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.312,18.0.737 definitions=2020-11-05_16:2020-11-05,2020-11-05 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 malwarescore=0 phishscore=0 clxscore=1015 lowpriorityscore=0 mlxlogscore=667 suspectscore=0 mlxscore=0 priorityscore=1501 impostorscore=0 bulkscore=0 adultscore=0 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011050158 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org > On Nov 5, 2020, at 4:06 PM, Daniel Xu wrote: >=20 > do_strncpy_from_user() may copy some extra bytes after the NUL > terminator into the destination buffer. This usually does not matter for > normal string operations. However, when BPF programs key BPF maps with > strings, this matters a lot. >=20 > A BPF program may read strings from user memory by calling the > bpf_probe_read_user_str() helper which eventually calls > do_strncpy_from_user(). The program can then key a map with the > resulting string. BPF map keys are fixed-width and string-agnostic, > meaning that map keys are treated as a set of bytes. >=20 > The issue is when do_strncpy_from_user() overcopies bytes after the NUL > terminator, it can result in seemingly identical strings occupying > multiple slots in a BPF map. This behavior is subtle and totally > unexpected by the user. >=20 > This commit uses the proper word-at-a-time APIs to avoid overcopying. >=20 > Fixes: 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{= user, kernel}_str helpers") > Signed-off-by: Daniel Xu Acked-by: Song Liu [...]