Re: [PATCH v5 05/10] powerpc: Add a framework for Kernel Userspace Access Protection

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 20/03/2019 à 13:57, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>> Le 08/03/2019 à 02:16, Michael Ellerman a écrit :
>>> From: Christophe Leroy <christophe.leroy@c-s.fr>
>>>
>>> This patch implements a framework for Kernel Userspace Access
>>> Protection.
>>>
>>> Then subarches will have the possibility to provide their own
>>> implementation by providing setup_kuap() and
>>> allow/prevent_user_access().
>>>
>>> Some platforms will need to know the area accessed and whether it is
>>> accessed from read, write or both. Therefore source, destination and
>>> size and handed over to the two functions.
>>>
>>> mpe: Rename to allow/prevent rather than unlock/lock, and add
>>> read/write wrappers. Drop the 32-bit code for now until we have an
>>> implementation for it. Add kuap to pt_regs for 64-bit as well as
>>> 32-bit. Don't split strings, use pr_crit_ratelimited().
>>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>> Signed-off-by: Russell Currey <ruscur@russell.cc>
>>> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
>>> ---
>>> v5: Futex ops need read/write so use allow_user_acccess() there.
>>>       Use #ifdef CONFIG_PPC64 in kup.h to fix build errors.
>>>       Allow subarch to override allow_read/write_from/to_user().
>>
>> Those little helpers that will just call allow_user_access() when
>> distinct read/write handling is not performed looks overkill to me.
>>
>> Can't the subarch do it by itself based on the nullity of from/to ?
>>
>> static inline void allow_user_access(void __user *to, const void __user
>> *from,
>> 				     unsigned long size)
>> {
>> 	if (to & from)
>> 		set_kuap(0);
>> 	else if (to)
>> 		set_kuap(AMR_KUAP_BLOCK_READ);
>> 	else if (from)
>> 		set_kuap(AMR_KUAP_BLOCK_WRITE);
>> }
> 
> You could implement it that way, but it reads better at the call sites
> if we have:
> 
> 	allow_write_to_user(uaddr, sizeof(*uaddr));
> vs:
> 	allow_user_access(uaddr, NULL, sizeof(*uaddr));
> 
> So I'm inclined to keep them. It should all end up inlined and generate
> the same code at the end of the day.
> 

I was not suggesting to completly remove allow_write_to_user(), I fully 
agree that it reads better at the call sites.

I was just thinking that allow_write_to_user() could remain generic and 
call the subarch specific allow_user_access() instead of making multiple 
subarch's allow_write_to_user()

But both solution are OK for me at the end.

Christophe