linux-kernel - Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

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Message-ID: <1ec0d015-90fa-e2f4-c9ef-e63ec43a46b3@redhat.com>
Date:   Wed, 25 Aug 2021 10:38:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables

On 24.08.21 01:50, Dave Hansen wrote:
> On 8/23/21 6:25 AM, Mike Rapoport wrote:
>>   void ___pte_free_tlb(struct mmu_gather *tlb, struct page *pte)
>>   {
>> +	enable_pgtable_write(page_address(pte));
>>   	pgtable_pte_page_dtor(pte);
>>   	paravirt_release_pte(page_to_pfn(pte));
>>   	paravirt_tlb_remove_table(tlb, pte);
>> @@ -69,6 +73,7 @@ void ___pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
>>   #ifdef CONFIG_X86_PAE
>>   	tlb->need_flush_all = 1;
>>   #endif
>> +	enable_pgtable_write(pmd);
>>   	pgtable_pmd_page_dtor(page);
>>   	paravirt_tlb_remove_table(tlb, page);
>>   }
> 
> I would expected this to have leveraged the pte_offset_map/unmap() code
> to enable/disable write access.  Granted, it would enable write access
> even when only a read is needed, but that could be trivially fixed with
> having a variant like:

For write access you actually want pte_offset_map_locked(), but it's 
also used for stable read access sometimes (exclude any writers).

> 
> 	pte_offset_map_write()
> 	pte_offset_unmap_write()
> 
> in addition to the existing (presumably read-only) versions:
> 
> 	pte_offset_map()
> 	pte_offset_unmap()

These should mostly only be read access, you'd need other ways of making 
sure nobody else messes with that entry. I think it even holds for 
khugepaged collapsing code.


I find these hidden PMD entry modifications (e.g., without holding the 
PMD lock) deep down in arch code quite concerning. Read: horribly ugly 
and a nightmare to debug.

-- 
Thanks,

David / dhildenb