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From: Dave Hansen <dave.hansen@intel.com>
To: Andy Lutomirski <luto@kernel.org>,
	Mike Rapoport <rppt@kernel.org>,
	linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.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 (Intel)" <peterz@infradead.org>,
	Rick P Edgecombe <rick.p.edgecombe@intel.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables
Date: Wed, 25 Aug 2021 07:59:49 -0700	[thread overview]
Message-ID: <a24105be-b07f-41e5-4c3e-0b8a911ce231@intel.com> (raw)
In-Reply-To: <8b50991d-2ab5-4577-83e9-a2d74135c5f5@www.fastmail.com>

On 8/23/21 8:34 PM, Andy Lutomirski wrote:
>> 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:
>>
>> 	pte_offset_map_write()
>> 	pte_offset_unmap_write()
> I would also like to see a discussion of how races in which multiple
> threads or CPUs access ptes in the same page at the same time.

Yeah, the 32-bit highmem code has a per-cpu mapping area for
kmap_atomic() that lies underneath the pte_offset_map().  Although it's
in the shared kernel mapping, only one CPU uses it.

I didn't look at it until you mentioned it, but the code in this set is
just plain buggy if two CPUs do a
enable_pgtable_write()/disable_pgtable_write().  They'll clobber each other:

> +static void pgtable_write_set(void *pg_table, bool set)
> +{
> +	int level = 0;
> +	pte_t *pte;
> +
> +	/*
> +	 * Skip the page tables allocated from pgt_buf break area and from
> +	 * memblock
> +	 */
> +	if (!after_bootmem)
> +		return;
> +	if (!PageTable(virt_to_page(pg_table)))
> +		return;
> +
> +	pte = lookup_address((unsigned long)pg_table, &level);
> +	if (!pte || level != PG_LEVEL_4K)
> +		return;
> +
> +	if (set) {
> +		if (pte_write(*pte))
> +			return;
> +
> +		WRITE_ONCE(*pte, pte_mkwrite(*pte));
> +	} else {
> +		if (!pte_write(*pte))
> +			return;
> +
> +		WRITE_ONCE(*pte, pte_wrprotect(*pte));
> +	}
> +}


  reply	other threads:[~2021-08-25 14:59 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 [this message]
2021-08-24 13:32     ` Mike Rapoport
2021-08-25  8:38     ` David Hildenbrand
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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