linux-kernel - Re: [PATCH 2/3] powerpc/book3e: Fix set_memory_x() and set_memory

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Date:   Wed, 27 Oct 2021 07:50:40 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/3] powerpc/book3e: Fix set_memory_x() and
 set_memory_nx()



Le 27/10/2021 à 07:27, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of October 27, 2021 2:55 pm:
>>
>>
>> Le 27/10/2021 à 06:44, Nicholas Piggin a écrit :
>>> Excerpts from Christophe Leroy's message of October 26, 2021 3:39 pm:
>>>> set_memory_x() calls pte_mkexec() which sets _PAGE_EXEC.
>>>> set_memory_nx() calls pte_exprotec() which clears _PAGE_EXEC.
>>>>
>>>> Book3e has 2 bits, UX and SX, which defines the exec rights
>>>> resp. for user (PR=1) and for kernel (PR=0).
>>>>
>>>> _PAGE_EXEC is defined as UX only.
>>>>
>>>> An executable kernel page is set with either _PAGE_KERNEL_RWX
>>>> or _PAGE_KERNEL_ROX, which both have SX set and UX cleared.
>>>>
>>>> So set_memory_nx() call for an executable kernel page does
>>>> nothing because UX is already cleared.
>>>>
>>>> And set_memory_x() on a non-executable kernel page makes it
>>>> executable for the user and keeps it non-executable for kernel.
>>>>
>>>> Also, pte_exec() always returns 'false' on kernel pages, because
>>>> it checks _PAGE_EXEC which doesn't include SX, so for instance
>>>> the W+X check doesn't work.
>>>>
>>>> To fix this:
>>>> - change tlb_low_64e.S to use _PAGE_BAP_UX instead of _PAGE_USER
>>>> - sets both UX and SX in _PAGE_EXEC so that pte_user() returns
>>>> true whenever one of the two bits is set
>>>
>>> I don't understand this change. Which pte_user() returns true after
>>> this change? Or do you mean pte_exec()?
>>
>> Oops, yes, I mean pte_exec()
>>
>> Unless I have to re-spin, can Michael eventually fix that typo while
>> applying ?
>>
>>>
>>> Does this filter through in some cases at least for kernel executable
>>> PTEs will get both bits set? Seems cleaner to distinguish user and
>>> kernel exec for that but maybe it's a lot of churn?
>>
>> Didn't understand what you mean.
>>
>> I did it like that to be able to continue using _PAGE_EXEC for checking
>> executability regardless of whether this is user or kernel, and then
>> continue using the generic nohash pte_exec() helper.
>>
>> Other solution would be to get rid of _PAGE_EXEC completely for book3e
>> and implement both pte_exec() and pte_mkexec() with _PAGE_BAP_UX and
>> _PAGE_BAP_SX, but I'm not sure it is worth the churn as you say. It
>> would also mean different helpers for book3s/32 when it is using 32 bits
>> PTE (CONFIG_PTE_64BIT=n)
> 
> That's basically what I mean. And _PAGE_KERNEL_ROX etc would then not
> set the UX bit. But at least for now it seems to be an improvement.
> 

That's already the case:

#define _PAGE_KERNEL_RWX	(_PAGE_BAP_SW | _PAGE_BAP_SR | _PAGE_DIRTY | 
_PAGE_BAP_SX)
#define _PAGE_KERNEL_ROX	(_PAGE_BAP_SR | _PAGE_BAP_SX)

Christophe