[PATCH] powerpc/mm: Fix KUAP warning by providing copy_from_kernel_nofault_allowed()

[Christophe Leroy]

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