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[79.242.61.107]) by smtp.gmail.com with ESMTPSA id 17sm4555894wmj.20.2021.08.25.01.38.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 25 Aug 2021 01:38:23 -0700 (PDT) To: Dave Hansen , Mike Rapoport , linux-mm@kvack.org Cc: Andrew Morton , Andy Lutomirski , Dave Hansen , Ira Weiny , Kees Cook , Mike Rapoport , Peter Zijlstra , Rick Edgecombe , Vlastimil Babka , x86@kernel.org, linux-kernel@vger.kernel.org References: <20210823132513.15836-1-rppt@kernel.org> <20210823132513.15836-5-rppt@kernel.org> <1cccc2b6-8b5b-4aee-483d-f10e64a248a5@intel.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [RFC PATCH 4/4] x86/mm: write protect (most) page tables Message-ID: <1ec0d015-90fa-e2f4-c9ef-e63ec43a46b3@redhat.com> Date: Wed, 25 Aug 2021 10:38:22 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <1cccc2b6-8b5b-4aee-483d-f10e64a248a5@intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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