linux-kernel - Re: [PATCH] powerpc/mm: Fix KUAP warning by providing copy_from_kernel_nofault

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Date:   Sat, 5 Dec 2020 13:38:49 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Christoph Hellwig <hch@....de>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] powerpc/mm: Fix KUAP warning by providing
 copy_from_kernel_nofault_allowed()



Le 05/12/2020 à 10:56, Christophe Leroy a écrit :
> 
> 
> Le 05/12/2020 à 09:48, Christoph Hellwig a écrit :
>> On Sat, Dec 05, 2020 at 08:43:06AM +0000, Christophe Leroy wrote:
>>> Since commit c33165253492 ("powerpc: use non-set_fs based maccess
>>> routines"), userspace access is not granted anymore when using
>>> copy_from_kernel_nofault()
>>>
>>> However, kthread_probe_data() uses copy_from_kernel_nofault()
>>> to check validity of pointers. When the pointer is NULL,
>>> it points to userspace, leading to a KUAP fault and triggering
>>> the following big hammer warning many times when you request
>>> a sysrq "show task":
>>
>>
>>
>>> To avoid that, copy_from_kernel_nofault_allowed() is used to check
>>> whether the address is a valid kernel address. But the default
>>> version of it returns true for any address.
>>>
>>> Provide a powerpc version of copy_from_kernel_nofault_allowed()
>>> that returns false when the address is below TASK_USER_MAX,
>>> so that copy_from_kernel_nofault() will return -ERANGE.
>>
>> Looks good.  I wonder if we should just default to the TASK_SIZE_MAX
>> check in  copy_from_kernel_nofault_allowed for architectures that select
>> CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE?
> 
> Yes maybe that would be better.
> 
> Can you cook a patch an get it into 5.10 ?
> 

In fact it doesn't seem so easy because only s390, powerpc and x86 have TASK_SIZE_MAX while 
CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE is selected by arm, arm64, powerpc and x86

So maybe for 5.10 we take the powerpc fix ?

Christophe