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Thu, 4 Feb 2021 17:02:08 +0000 (GMT) Received: from [9.102.0.141] (unknown [9.102.0.141]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Thu, 4 Feb 2021 17:02:08 +0000 (GMT) Subject: Re: [PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING To: Jens Axboe , Christophe Leroy , Nicholas Piggin , Michael Ellerman , Zorro Lang References: <20210127145648.348135-1-zlang@redhat.com> <1611792928.nw4g8h8kj4.astroid@bobo.none> <20210128031355.GP14354@localhost.localdomain> <66061f75-c8de-c1eb-aaaf-9594a31be790@kernel.dk> <20210128135220.GQ14354@localhost.localdomain> <17ae2706-fe95-a5de-b9da-e3480800daf7@csgroup.eu> <20210129065220.GS14354@localhost.localdomain> <18dd441b-440a-fe95-0907-d8cec5b49410@csgroup.eu> <87r1m2d5z4.fsf@mpe.ellerman.id.au> <1612014260.b4fac0liie.astroid@bobo.none> <877dnrrsbu.fsf@linux.ibm.com> <874kivrp2v.fsf@linux.ibm.com> <6b081ea7-e4ee-21bb-7085-e33b4e5c6205@csgroup.eu> <7c48c517-700d-e114-503d-e68e0e73c534@linux.ibm.com> <7c06ba68-9959-44bc-233b-473d7cbc574a@csgroup.eu> <086f6525-b851-c4c0-d611-42f76a54a2d9@kernel.dk> From: "Aneesh Kumar K.V" Message-ID: <490cea1a-8161-2f76-03e6-7e2b16efa648@linux.ibm.com> Date: Thu, 4 Feb 2021 22:31:59 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1 MIME-Version: 1.0 In-Reply-To: <086f6525-b851-c4c0-d611-42f76a54a2d9@kernel.dk> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369, 18.0.737 definitions=2021-02-04_08:2021-02-04, 2021-02-04 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 mlxscore=0 priorityscore=1501 impostorscore=0 malwarescore=0 spamscore=0 adultscore=0 phishscore=0 bulkscore=0 clxscore=1015 mlxlogscore=999 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2102040101 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 2/4/21 10:23 PM, Jens Axboe wrote: > On 2/1/21 11:30 PM, Aneesh Kumar K.V wrote: >> On 2/2/21 11:50 AM, Christophe Leroy wrote: >>> >>> >>> Le 02/02/2021 à 07:16, Aneesh Kumar K.V a écrit : >>>> On 2/2/21 11:32 AM, Christophe Leroy wrote: >>>>> >>>>> >>>>> Le 02/02/2021 à 06:55, Aneesh Kumar K.V a écrit : >>>>>> Aneesh Kumar K.V writes: >>>>>> >>>>>>> Nicholas Piggin writes: >>>>>>> >>>>>>>> Excerpts from Michael Ellerman's message of January 30, 2021 9:22 pm: >>>>>>>>> Christophe Leroy writes: >>>>>>>>>> +Aneesh >>>>>>>>>> >>>>>>>>>> Le 29/01/2021 à 07:52, Zorro Lang a écrit : >>>>>>>>> .. >>>>>>>>>>> [ 96.200296] ------------[ cut here ]------------ >>>>>>>>>>> [ 96.200304] Bug: Read fault blocked by KUAP! >>>>>>>>>>> [ 96.200309] WARNING: CPU: 3 PID: 1876 at >>>>>>>>>>> arch/powerpc/mm/fault.c:229 bad_kernel_fault+0x180/0x310 >>>>>>>>>> >>>>>>>>>>> [ 96.200734] NIP [c000000000849424] >>>>>>>>>>> fault_in_pages_readable+0x104/0x350 >>>>>>>>>>> [ 96.200741] LR [c00000000084952c] >>>>>>>>>>> fault_in_pages_readable+0x20c/0x350 >>>>>>>>>>> [ 96.200747] --- interrupt: 300 >>>>>>>>>> >>>>>>>>>> >>>>>>>>>> Problem happens in a section where userspace access is supposed >>>>>>>>>> to be granted, so the patch you >>>>>>>>>> proposed is definitely not the right fix. >>>>>>>>>> >>>>>>>>>> c000000000849408: 2c 01 00 4c isync >>>>>>>>>> c00000000084940c: a6 03 3d 7d mtspr 29,r9 <== granting >>>>>>>>>> userspace access permission >>>>>>>>>> c000000000849410: 2c 01 00 4c isync >>>>>>>>>> c000000000849414: 00 00 36 e9 ld r9,0(r22) >>>>>>>>>> c000000000849418: 20 00 29 81 lwz r9,32(r9) >>>>>>>>>> c00000000084941c: 00 02 29 71 andi. r9,r9,512 >>>>>>>>>> c000000000849420: 78 d3 5e 7f mr r30,r26 >>>>>>>>>> ==> c000000000849424: 00 00 bf 8b lbz r29,0(r31) <== >>>>>>>>>> accessing userspace >>>>>>>>>> c000000000849428: 10 00 82 41 beq c000000000849438 >>>>>>>>>> >>>>>>>>>> c00000000084942c: 2c 01 00 4c isync >>>>>>>>>> c000000000849430: a6 03 bd 7e mtspr 29,r21 <== >>>>>>>>>> clearing userspace access permission >>>>>>>>>> c000000000849434: 2c 01 00 4c isync >>>>>>>>>> >>>>>>>>>> My first guess is that the problem is linked to the following >>>>>>>>>> function, see the comment >>>>>>>>>> >>>>>>>>>> /* >>>>>>>>>> * For kernel thread that doesn't have thread.regs return >>>>>>>>>> * default AMR/IAMR values. >>>>>>>>>> */ >>>>>>>>>> static inline u64 current_thread_amr(void) >>>>>>>>>> { >>>>>>>>>> if (current->thread.regs) >>>>>>>>>> return current->thread.regs->amr; >>>>>>>>>> return AMR_KUAP_BLOCKED; >>>>>>>>>> } >>>>>>>>>> >>>>>>>>>> Above function was introduced by commit 48a8ab4eeb82 >>>>>>>>>> ("powerpc/book3s64/pkeys: Don't update SPRN_AMR >>>>>>>>>> when in kernel mode") >>>>>>>>> >>>>>>>>> Yeah that's a bit of a curly one. >>>>>>>>> >>>>>>>>> At some point io_uring did kthread_use_mm(), which is supposed to >>>>>>>>> mean >>>>>>>>> the kthread can operate on behalf of the original process that >>>>>>>>> submitted >>>>>>>>> the IO. >>>>>>>>> >>>>>>>>> But because KUAP is implemented using memory protection keys, it >>>>>>>>> depends >>>>>>>>> on the value of the AMR register, which is not part of the mm, >>>>>>>>> it's in >>>>>>>>> thread.regs->amr. >>>>>>>>> >>>>>>>>> And what's worse by the time we're in kthread_use_mm() we no >>>>>>>>> longer have >>>>>>>>> access to the thread.regs->amr of the original process that >>>>>>>>> submitted >>>>>>>>> the IO. >>>>>>>>> >>>>>>>>> We also can't simply move the AMR into the mm, precisely because >>>>>>>>> it's >>>>>>>>> per thread, not per mm. >>>>>>>>> >>>>>>>>> So TBH I don't know how we're going to fix this. >>>>>>>>> >>>>>>>>> I guess we could return AMR=unblocked for kernel threads, but that's >>>>>>>>> arguably a bug because it allows a process to circumvent memory >>>>>>>>> keys by >>>>>>>>> asking the kernel to do the access. >>>>>>>> >>>>>>>> We shouldn't need to inherit AMR should we? We only need it to be >>>>>>>> locked >>>>>>>> for kernel threads until it's explicitly unlocked -- nothing mm >>>>>>>> specific >>>>>>>> there. I think current_thread_amr could return 0 for kernel >>>>>>>> threads? Or >>>>>>>> I would even avoid using that function for allow_user_access and open >>>>>>>> code the kthread case and remove it from current_thread_amr(). >>>>>>>> >>>>>>>> Thanks, >>>>>>>> Nick >>>>>>> >>>>>> >>>>>> updated one >>>>>> >>>>>> From 8fdb0680f983940d61f91da8252b13c8d3e8ebee Mon Sep 17 00:00:00 2001 >>>>>> From: "Aneesh Kumar K.V" >>>>>> Date: Tue, 2 Feb 2021 09:23:38 +0530 >>>>>> Subject: [PATCH v2] powerpc/kuap: Allow kernel thread to access >>>>>> userspace >>>>>> after kthread_use_mm >>>>>> >>>>>> This fix the bad fault reported by KUAP when io_wqe_worker access >>>>>> userspace. >>>>>> >>>>>> Bug: Read fault blocked by KUAP! >>>>>> WARNING: CPU: 1 PID: 101841 at arch/powerpc/mm/fault.c:229 >>>>>> __do_page_fault+0x6b4/0xcd0 >>>>>> NIP [c00000000009e7e4] __do_page_fault+0x6b4/0xcd0 >>>>>> LR [c00000000009e7e0] __do_page_fault+0x6b0/0xcd0 >>>>>> .......... >>>>>> Call Trace: >>>>>> [c000000016367330] [c00000000009e7e0] __do_page_fault+0x6b0/0xcd0 >>>>>> (unreliable) >>>>>> [c0000000163673e0] [c00000000009ee3c] do_page_fault+0x3c/0x120 >>>>>> [c000000016367430] [c00000000000c848] handle_page_fault+0x10/0x2c >>>>>> --- interrupt: 300 at iov_iter_fault_in_readable+0x148/0x6f0 >>>>>> .......... >>>>>> NIP [c0000000008e8228] iov_iter_fault_in_readable+0x148/0x6f0 >>>>>> LR [c0000000008e834c] iov_iter_fault_in_readable+0x26c/0x6f0 >>>>>> interrupt: 300 >>>>>> [c0000000163677e0] [c0000000007154a0] iomap_write_actor+0xc0/0x280 >>>>>> [c000000016367880] [c00000000070fc94] iomap_apply+0x1c4/0x780 >>>>>> [c000000016367990] [c000000000710330] >>>>>> iomap_file_buffered_write+0xa0/0x120 >>>>>> [c0000000163679e0] [c00800000040791c] >>>>>> xfs_file_buffered_aio_write+0x314/0x5e0 [xfs] >>>>>> [c000000016367a90] [c0000000006d74bc] io_write+0x10c/0x460 >>>>>> [c000000016367bb0] [c0000000006d80e4] io_issue_sqe+0x8d4/0x1200 >>>>>> [c000000016367c70] [c0000000006d8ad0] io_wq_submit_work+0xc0/0x250 >>>>>> [c000000016367cb0] [c0000000006e2578] >>>>>> io_worker_handle_work+0x498/0x800 >>>>>> [c000000016367d40] [c0000000006e2cdc] io_wqe_worker+0x3fc/0x4f0 >>>>>> [c000000016367da0] [c0000000001cb0a4] kthread+0x1c4/0x1d0 >>>>>> [c000000016367e10] [c00000000000dbf0] >>>>>> ret_from_kernel_thread+0x5c/0x6c >>>>>> >>>>>> The kernel consider thread AMR value for kernel thread to be >>>>>> AMR_KUAP_BLOCKED. Hence access to userspace is denied. This >>>>>> of course not correct and we should allow userspace access after >>>>>> kthread_use_mm(). To be precise, kthread_use_mm() should inherit the >>>>>> AMR value of the operating address space. But, the AMR value is >>>>>> thread-specific and we inherit the address space and not thread >>>>>> access restrictions. Because of this ignore AMR value when accessing >>>>>> userspace via kernel thread. >>>>>> >>>>>> Signed-off-by: Aneesh Kumar K.V >>>>>> --- >>>>>> Changes from v1: >>>>>> * Address review feedback from Nick >>>>>> >>>>>> arch/powerpc/include/asm/book3s/64/kup.h | 8 +++++++- >>>>>> 1 file changed, 7 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/arch/powerpc/include/asm/book3s/64/kup.h >>>>>> b/arch/powerpc/include/asm/book3s/64/kup.h >>>>>> index f50f72e535aa..95f4df99249e 100644 >>>>>> --- a/arch/powerpc/include/asm/book3s/64/kup.h >>>>>> +++ b/arch/powerpc/include/asm/book3s/64/kup.h >>>>>> @@ -384,7 +384,13 @@ static __always_inline void >>>>>> allow_user_access(void __user *to, const void __user >>>>>> // This is written so we can resolve to a single case at build >>>>>> time >>>>>> BUILD_BUG_ON(!__builtin_constant_p(dir)); >>>>>> - if (mmu_has_feature(MMU_FTR_PKEY)) >>>>>> + /* >>>>>> + * if it is a kthread that did kthread_use_mm() don't >>>>>> + * use current_thread_amr(). >>>>> >>>>> According to include/linux/sched.h, PF_KTHREAD means /* I am a kernel >>>>> thread */ >>>>> It doesn't seem to be related to kthread_use_mm() >>>> >>>> That should be a sufficient check here. if we did reach here without >>>> calling kthread_user_mm, we will crash on access because we don't have >>>> a mm attached to the current process. a kernel thread with >>>> kthread_use_mm has >>> >>> Ok but then the comment doesn't match the check. >> >> >> I was trying to be explict in the comment that we expect the thread to >> have done kthread_use_mm(). >> >>> >>> And also the comment in current_thread_amr() is then misleading. >>> >>> Why not do the current->flags & PF_KTHREAD check in current_thread_amr() >>> and return 0 in that case instead of BLOCKED ? >> >> In my view currrent_thread_amr() is more generic and we want to be >> explicit there that a kernel thread AMR is KUAP_BLOCKED. Only when we >> call allow user access, we relax the AMR value. > > Just following up on this, as I'd hate to have 5.11 released with this > bug in it for powerpc. It'll obviously also affect other cases of a > kernel thread faulting after having done kthread_use_mm(), though I'm > not sure how widespread that is. In any case, it'll leave io_uring > mostly broken on powerpc if this isn't patched for release. > I am waiting for test feedback on the change I posted earlier. I am also running a regression run myself. Once that is complete i will post the patch as a separate email. -aneesh