[PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING
Aneesh Kumar K.V
aneesh.kumar at linux.ibm.com
Fri Feb 5 04:01:59 AEDT 2021
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 <aneesh.kumar at linux.ibm.com> writes:
>>>>>>
>>>>>>> Nicholas Piggin <npiggin at gmail.com> writes:
>>>>>>>
>>>>>>>> Excerpts from Michael Ellerman's message of January 30, 2021 9:22 pm:
>>>>>>>>> Christophe Leroy <christophe.leroy at csgroup.eu> 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
>>>>>>>>>> <fault_in_pages_readable+0x118>
>>>>>>>>>> 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" <aneesh.kumar at linux.ibm.com>
>>>>>> 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 <aneesh.kumar at linux.ibm.com>
>>>>>> ---
>>>>>> 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
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