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

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Message-ID: <063e72e1-fc05-7783-9f42-f681dd08a4b2@csgroup.eu>
Date:   Wed, 27 Oct 2021 06:55:32 +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 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)

Christophe

> 
> Thanks,
> Nick
> 
>> and pte_exprotect()
>> clears both bits.
>> - Define a book3e specific version of pte_mkexec() which sets
>> either SX or UX based on UR.
>>
>> Fixes: 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines")
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>> v3: Removed pte_mkexec() from nohash/64/pgtable.h
>> v2: New
>> ---