Subject: Re: [PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 2 Feb 2021 11:46:37 +0530

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 <[email-masked]> writes:
>>
>>> Nicholas Piggin <[email-masked]> writes:
>>>
>>>> Excerpts from Michael Ellerman's message of January 30, 2021 9:22 pm:
>>>>> Christophe Leroy <[email-masked]> 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" <[email-masked]>
>> 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 <[email-masked]>
>> ---
>> 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

current->mm == current->active_mm && current->flags & PF_KTHREAD.

The first part is true for every other process too.

> 
>> +     */
>> +    if (current->flags & PF_KTHREAD)
>> +        thread_amr = 0;
>> +    else if (mmu_has_feature(MMU_FTR_PKEY))
>>           thread_amr = current_thread_amr();
>>       if (dir == KUAP_READ)
>>
> 
> Christophe

-aneesh