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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 02 Feb 2021 11:25:20 +0530

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().
+	 */
+	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