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

Christophe Leroy christophe.leroy at csgroup.eu
Tue Feb 2 17:20:43 AEDT 2021



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.

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 ?

> 
> 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


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