[PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING
Aneesh Kumar K.V
aneesh.kumar at linux.ibm.com
Tue Feb 2 17:16:37 AEDT 2021
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
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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