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
