Excerpts from Aneesh Kumar K.V's message of February 2, 2021 4:30 pm:
> 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.ku...@linux.ibm.com> writes:
>>>>>
>>>>>> Nicholas Piggin <npig...@gmail.com> writes:
>>>>>>
>>>>>>> Excerpts from Michael Ellerman's message of January 30, 2021 9:22 pm:
>>>>>>>> Christophe Leroy <christophe.le...@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.ku...@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.ku...@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().
I would avoid making it sound conditional. There is no way for a kernel
thread to ever access user without having done so.
/*
* Kernel threads may access user mm with kthread_use_mm() but
* can't use current_thread_amr because they have thread.regs==NULL,
* but they have no pkeys.
*/
>
>>
>> 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.
current_thread_amr() shouldn't be used by kernel threads ever
(after the patch). It's just confusing. The user access or the
check could have a test and warning for !current->mm if there
is a concern about it (maybe there already is one somehwere).
Thanks,
Nick
