[PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING
Aneesh Kumar K.V
aneesh.kumar at linux.ibm.com
Fri Feb 5 04:42:47 AEDT 2021
On 2/4/21 11:27 PM, Zorro Lang wrote:
> On Thu, Feb 04, 2021 at 10:31:59PM +0530, Aneesh Kumar K.V wrote:
>> 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.
>
> Are you waiting a test from me? Or someone else who test PPC? Although I'm
> the "Reported-by" of this bug, I just can help to verify this bug itself,
> I don't have enough test cases to do regression test from PPC side. Do you need
> me to verify this bug itself.
>
>
If you can verify this bug, it would be really helpful.
-aneesh
More information about the Linuxppc-dev
mailing list