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

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Message-Id: <1635312355.da7w1oggf1.astroid@bobo.none>
Date:   Wed, 27 Oct 2021 15:27:21 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        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()

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.

Thanks,
Nick