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([2601:647:5380:8c0:858e:9ae5:3ea5:20eb]) by smtp.gmail.com with ESMTPSA id t19sm6195113pfg.70.2021.05.29.00.31.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 29 May 2021 00:31:44 -0700 (PDT) Subject: Re: Sealed memfd & no-fault mmap To: Linus Torvalds , Hugh Dickins Cc: Simon Ser , Peter Xu , "Kirill A. Shutemov" , Matthew Wilcox , Dan Williams , "Kirill A. Shutemov" , Will Deacon , Linux Kernel Mailing List , David Herrmann , "linux-mm@kvack.org" , Greg Kroah-Hartman , "tytso@mit.edu" References: <20210429154807.hptls4vnmq2svuea@box> <20210429183836.GF8339@xz-x1> <7718ec5b-0a9e-ffa6-16f2-bc0b6afbd9ab@gmail.com> From: "Lin, Ming" Message-ID: <80c87e6b-6050-bf23-2185-ded408df4d0f@gmail.com> Date: Sat, 29 May 2021 00:31:39 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/28/2021 6:03 PM, Linus Torvalds wrote: > On Fri, May 28, 2021 at 7:07 AM Lin, Ming wrote: >> >> Does something like following draft patch on the right track? > > No, I don't think this can work: > >> + _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr), >> + vma->vm_page_prot)); > > You can't just blindly insert the zero pfn - for a shared write > mapping, that would actually allow writes to the zeropage. That would > be horrible. I should check the vma is not writable. diff --git a/mm/shmem.c b/mm/shmem.c index 856d2d8d4cdf..fa23e38bc692 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1820,7 +1820,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index, spinlock_t *ptl; int ret; - if (!IS_NOFAULT(inode)) + if (!IS_NOFAULT(inode) || (vma->vm_flags & VM_WRITE)) return -EINVAL; _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr)