[PATCH] powerpc/fault: fix wrong KUAP fault for IO_URING
Aneesh Kumar K.V
aneesh.kumar at linux.ibm.com
Tue Feb 2 16:55:20 AEDT 2021
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().
+ */
+ 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)
--
2.29.2
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