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From: David Hildenbrand <david@redhat.com>
To: Dave Hansen <dave.hansen@intel.com>,
	Mike Rapoport <rppt@kernel.org>,
	linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Kees Cook <keescook@chromium.org>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables
Date: Wed, 25 Aug 2021 10:38:22 +0200	[thread overview]
Message-ID: <1ec0d015-90fa-e2f4-c9ef-e63ec43a46b3@redhat.com> (raw)
In-Reply-To: <1cccc2b6-8b5b-4aee-483d-f10e64a248a5@intel.com>

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


  parent reply	other threads:[~2021-08-25  8:38 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23 13:25 [RFC PATCH 0/4] mm/page_alloc: cache pte-mapped allocations Mike Rapoport
2021-08-23 13:25 ` [RFC PATCH 1/4] list: Support getting most recent element in list_lru Mike Rapoport
2021-08-23 13:25 ` [RFC PATCH 2/4] list: Support list head not in object for list_lru Mike Rapoport
2021-08-23 13:25 ` [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE_MAPPED flag to allocate pte-mapped pages Mike Rapoport
2021-08-23 20:29   ` Edgecombe, Rick P
2021-08-24 13:02     ` Mike Rapoport
2021-08-24 16:38       ` Edgecombe, Rick P
2021-08-24 16:54         ` Mike Rapoport
2021-08-24 17:23           ` Edgecombe, Rick P
2021-08-24 17:37             ` Mike Rapoport
2021-08-24 16:12   ` Vlastimil Babka
2021-08-25  8:43   ` David Hildenbrand
2021-08-23 13:25 ` [RFC PATCH 4/4] x86/mm: write protect (most) page tables Mike Rapoport
2021-08-23 20:08   ` Edgecombe, Rick P
2021-08-23 23:50   ` Dave Hansen
2021-08-24  3:34     ` Andy Lutomirski
2021-08-25 14:59       ` Dave Hansen
2021-08-24 13:32     ` Mike Rapoport
2021-08-25  8:38     ` David Hildenbrand [this message]
2021-08-26  8:02     ` Mike Rapoport
2021-08-26  9:01       ` Vlastimil Babka
     [not found]   ` <FB6C09CD-9CEA-4FE8-B179-98DB63EBDD68@gmail.com>
2021-08-24  5:34     ` Nadav Amit
2021-08-24 13:36       ` Mike Rapoport
2021-08-23 20:02 ` [RFC PATCH 0/4] mm/page_alloc: cache pte-mapped allocations Edgecombe, Rick P
2021-08-24 13:03   ` Mike Rapoport
2021-08-24 16:09 ` Vlastimil Babka
2021-08-29  7:06   ` Mike Rapoport

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