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Thu, 5 Nov 2020 18:18:43 +0000 From: Song Liu To: Daniel Xu CC: bpf , "linux-kernel@vger.kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , Kernel Team Subject: Re: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator Thread-Topic: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes after NUL terminator Thread-Index: AQHWsxsGDNyincyzREiD40J5/8I0oqm52XIAgAAAhgA= Date: Thu, 5 Nov 2020 18:18:43 +0000 Message-ID: <7DA54E81-F1FD-4CF7-946A-5B629B56C4A5@fb.com> References: <487a07aa911b4e822a0b931f7b33a4f67fedb6bd.1604542786.git.dxu@dxuuu.xyz> In-Reply-To: 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: 0233d72d-e646-480c-3cc2-08d881b73a75 x-ms-traffictypediagnostic: BYAPR15MB2568: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:1013; 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charset="us-ascii" Content-ID: <258FD37F3A020348808839C251152AD6@namprd15.prod.outlook.com> 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: 0233d72d-e646-480c-3cc2-08d881b73a75 X-MS-Exchange-CrossTenant-originalarrivaltime: 05 Nov 2020 18:18:43.1625 (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: TfhizBgGj0nP37p8wQ3zk87bTB5I8m53aPtEWmiXNOGnRYTqlFdAHlVxMnaRjSGaAi5cHhyf9TeCVO4ZI7qLPg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2568 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.312,18.0.737 definitions=2020-11-05_11:2020-11-05,2020-11-05 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 bulkscore=0 priorityscore=1501 phishscore=0 suspectscore=0 impostorscore=0 lowpriorityscore=0 adultscore=0 malwarescore=0 spamscore=0 mlxscore=0 mlxlogscore=999 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2011050118 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Nov 5, 2020, at 10:16 AM, Song Liu wrote: >=20 >=20 >=20 >> On Nov 4, 2020, at 6:25 PM, Daniel Xu wrote: >>=20 >> do_strncpy_from_user() may copy some extra bytes after the NUL >=20 > We have multiple use of "NUL" here, should be "NULL"? Just realized strncpy_from_user.c uses "NUL", so nevermind... >=20 >> 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 >> --- >> lib/strncpy_from_user.c | 9 +++++++-- >> 1 file changed, 7 insertions(+), 2 deletions(-) >>=20 >> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c >> index e6d5fcc2cdf3..d084189eb05c 100644 >> --- a/lib/strncpy_from_user.c >> +++ b/lib/strncpy_from_user.c >> @@ -35,17 +35,22 @@ static inline long do_strncpy_from_user(char *dst, c= onst char __user *src, >> goto byte_at_a_time; >>=20 >> while (max >=3D sizeof(unsigned long)) { >> - unsigned long c, data; >> + unsigned long c, data, mask, *out; >>=20 >> /* Fall back to byte-at-a-time if we get a page fault */ >> unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time); >>=20 >> - *(unsigned long *)(dst+res) =3D c; >> if (has_zero(c, &data, &constants)) { >> data =3D prep_zero_mask(c, data, &constants); >> data =3D create_zero_mask(data); >> + mask =3D zero_bytemask(data); >> + out =3D (unsigned long *)(dst+res); >> + *out =3D (*out & ~mask) | (c & mask); >> return res + find_zero(data); >> + } else { >=20 > This else clause is not needed, as we return in the if clause.=20 >=20 >> + *(unsigned long *)(dst+res) =3D c; >> } >> + >> res +=3D sizeof(unsigned long); >> max -=3D sizeof(unsigned long); >> } >> --=20 >> 2.28.0