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Fri, 29 May 2020 18:41:18 +0000 Subject: Re: [bpf PATCH 2/3] bpf, selftests: verifier bounds tests need to be updated To: John Fastabend , , CC: , References: <159077324869.6014.6516130782021506562.stgit@john-Precision-5820-Tower> <159077333942.6014.14004320043595756079.stgit@john-Precision-5820-Tower> From: Yonghong Song Message-ID: Date: Fri, 29 May 2020 11:41:16 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Thunderbird/68.8.1 In-Reply-To: <159077333942.6014.14004320043595756079.stgit@john-Precision-5820-Tower> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: BYAPR11CA0097.namprd11.prod.outlook.com (2603:10b6:a03:f4::38) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21cf::1387] (2620:10d:c090:400::5:c583) by BYAPR11CA0097.namprd11.prod.outlook.com (2603:10b6:a03:f4::38) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3045.19 via Frontend Transport; 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X-MS-Exchange-AntiSpam-MessageData: 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 X-MS-Exchange-CrossTenant-Network-Message-Id: dc757de5-19c5-43e9-57c3-08d803ffdfc6 X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 May 2020 18:41:18.1745 (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: DhCzB53MWCzyJlFLE0+swm1MvURBbaKJVQBqASL0HqyvgTTTutDZPTiDawCOl55S X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB2327 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216,18.0.687 definitions=2020-05-29_10:2020-05-28,2020-05-29 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 bulkscore=0 adultscore=0 mlxlogscore=999 mlxscore=0 priorityscore=1501 phishscore=0 clxscore=1015 lowpriorityscore=0 cotscore=-2147483648 spamscore=0 malwarescore=0 suspectscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2005290140 X-FB-Internal: deliver Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 5/29/20 10:28 AM, John Fastabend wrote: > After previous fix for zero extension test_verifier tests #65 and #66 now > fail. Before the fix we can see the alu32 mov op at insn 10 > > 10: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=invP(id=0, > smin_value=4294967168,smax_value=4294967423, > umin_value=4294967168,umax_value=4294967423, > var_off=(0x0; 0x1ffffffff), > s32_min_value=-2147483648,s32_max_value=2147483647, > u32_min_value=0,u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > 10: (bc) w1 = w1 > 11: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=invP(id=0, > smin_value=0,smax_value=2147483647, > umin_value=0,umax_value=4294967295, > var_off=(0x0; 0xffffffff), > s32_min_value=-2147483648,s32_max_value=2147483647, > u32_min_value=0,u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > > After the fix at insn 10 because we have 's32_min_value < 0' the following > step 11 now has 'smax_value=U32_MAX' where before we pulled the s32_max_value > bound into the smax_value as seen above in 11 with smax_value=2147483647. > > 10: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=inv(id=0, > smin_value=4294967168,smax_value=4294967423, > umin_value=4294967168,umax_value=4294967423, > var_off=(0x0; 0x1ffffffff), > s32_min_value=-2147483648, s32_max_value=2147483647, > u32_min_value=0,u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > 10: (bc) w1 = w1 > 11: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=inv(id=0, > smin_value=0,smax_value=4294967295, > umin_value=0,umax_value=4294967295, > var_off=(0x0; 0xffffffff), > s32_min_value=-2147483648, s32_max_value=2147483647, > u32_min_value=0, u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > > The fall out of this is by the time we get to the failing instruction at > step 14 where previously we had the following: > > 14: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=inv(id=0, > smin_value=72057594021150720,smax_value=72057594029539328, > umin_value=72057594021150720,umax_value=72057594029539328, > var_off=(0xffffffff000000; 0xffffff), > s32_min_value=-16777216,s32_max_value=-1, > u32_min_value=-16777216,u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > 14: (0f) r0 += r1 > > We now have, > > 14: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=inv(id=0, > smin_value=0,smax_value=72057594037927935, > umin_value=0,umax_value=72057594037927935, > var_off=(0x0; 0xffffffffffffff), > s32_min_value=-2147483648,s32_max_value=2147483647, > u32_min_value=0,u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > 14: (0f) r0 += r1 > > In the original step 14 'smin_value=72057594021150720' this trips the logic > in the verifier function check_reg_sane_offset(), > > if (smin >= BPF_MAX_VAR_OFF || smin <= -BPF_MAX_VAR_OFF) { > verbose(env, "value %lld makes %s pointer be out of bounds\n", > smin, reg_type_str[type]); > return false; > } > > Specifically, the 'smin <= -BPF_MAX_VAR_OFF' check. But with the fix > at step 14 we have bounds 'smin_value=0' so the above check is not tripped > because BPF_MAX_VAR_OFF=1<<29. > > We have a smin_value=0 here because at step 10 the smaller smin_value=0 means > the subtractions at steps 11 and 12 bring the smin_value negative. > > 11: (17) r1 -= 2147483584 > 12: (17) r1 -= 2147483584 > 13: (77) r1 >>= 8 > > Then the shift clears the top bit and smin_value is set to 0. Note we still > have the smax_value in the fixed code so any reads will fail. An alternative > would be to have reg_sane_check() do both smin and smax value tests. > > To fix the test we can omit the 'r1 >>=8' at line 13. This will change the > err string, but keeps the intention of the test as suggseted by the title, > "check after truncation of boundary-crossing range". If the verifier logic > changes a different value is likely to be thrown in the error or the error > will no longer be thrown forcing this test to be examined. With this change > we see the new state at step 13. > > 13: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0) > R1_w=invP(id=0, > smin_value=-4294967168,smax_value=127, > umin_value=0,umax_value=18446744073709551615, > s32_min_value=-2147483648,s32_max_value=2147483647, > u32_min_value=0,u32_max_value=-1) > R10=fp0 fp-8_w=mmmmmmmm > > Giving the expected out of bounds error, "value -4294967168 makes map_value > pointer be out of bounds" However, for unpriv case we see a different error > now because of the mixed signed bounds pointer arithmatic. This seems OK so > I've only added the unpriv_errstr for this. Another optino may have been to > do addition on r1 instead of subtraction but I favor the approach above > slightly. > > Signed-off-by: John Fastabend > --- > tools/testing/selftests/bpf/verifier/bounds.c | 24 ++++++++++-------------- > 1 file changed, 10 insertions(+), 14 deletions(-) Acked-by: Yonghong Song