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[2003:cb:c704:7800:3f13:77ac:9360:5e22]) by smtp.gmail.com with ESMTPSA id t20-20020a05600c199400b003cf9bf5208esm10877851wmq.19.2022.11.07.11.27.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 07 Nov 2022 11:27:11 -0800 (PST) Message-ID: Date: Mon, 7 Nov 2022 20:27:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH RFC 05/19] mm: add early FAULT_FLAG_WRITE consistency checks To: Nadav Amit Cc: kernel list , Linux-MM , "etnaviv@lists.freedesktop.org" , "dri-devel@lists.freedesktop.org" , "linux-arm-kernel@lists.infradead.org" , "linux-samsung-soc@vger.kernel.org" , "linux-rdma@vger.kernel.org" , "linux-media@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , Linus Torvalds , Andrew Morton , Jason Gunthorpe , John Hubbard , Peter Xu , Greg Kroah-Hartman , Andrea Arcangeli , Hugh Dickins , Vlastimil Babka , Matthew Wilcox , Mike Kravetz , Muchun Song , Lucas Stach , David Airlie , Oded Gabbay , Arnd Bergmann References: <20221107161740.144456-1-david@redhat.com> <20221107161740.144456-6-david@redhat.com> Content-Language: en-US From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org On 07.11.22 20:03, Nadav Amit wrote: > On Nov 7, 2022, at 8:17 AM, David Hildenbrand wrote: > >> !! External Email >> >> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to >> care in all other handlers and might get "surprises" if we forget to do >> so. >> >> Write faults without VM_MAYWRITE don't make any sense, and our >> maybe_mkwrite() logic could have hidden such abuse for now. >> >> Write faults without VM_WRITE on something that is not a COW mapping is >> similarly broken, and e.g., do_wp_page() could end up placing an >> anonymous page into a shared mapping, which would be bad. >> >> This is a preparation for reliable R/O long-term pinning of pages in >> private mappings, whereby we want to make sure that we will never break >> COW in a read-only private mapping. >> >> Signed-off-by: David Hildenbrand >> --- >> mm/memory.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index fe131273217a..826353da7b23 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -5159,6 +5159,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma, >> */ >> if (!is_cow_mapping(vma->vm_flags)) >> *flags &= ~FAULT_FLAG_UNSHARE; >> + } else if (*flags & FAULT_FLAG_WRITE) { >> + /* Write faults on read-only mappings are impossible ... */ >> + if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE))) >> + return VM_FAULT_SIGSEGV; >> + /* ... and FOLL_FORCE only applies to COW mappings. */ >> + if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) && >> + !is_cow_mapping(vma->vm_flags))) >> + return VM_FAULT_SIGSEGV; > > Not sure about the WARN_*(). Seems as if it might trigger in benign even if > rare scenarios, e.g., mprotect() racing with page-fault. > We most certainly would want to catch any such broken/racy cases. There are no benign cases I could possibly think of. Page faults need the mmap lock in read. mprotect() / VMA changes need the mmap lock in write. Whoever calls handle_mm_fault() is supposed to properly check VMA permissions. -- Thanks, David / dhildenb