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[2003:cb:c702:dd00:745e:20b7:bfa4:2e5f]) by smtp.gmail.com with ESMTPSA id u5-20020adfed45000000b00207a480e6aasm2161989wro.116.2022.04.22.10.41.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 22 Apr 2022 10:41:38 -0700 (PDT) Message-ID: <8aafe4ad-81a6-31a5-c95a-ce2943bf0525@redhat.com> Date: Fri, 22 Apr 2022 19:41:37 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag Content-Language: en-US To: Catalin Marinas Cc: Andrew Morton , Christoph Hellwig , Lennart Poettering , =?UTF-8?Q?Zbigniew_J=c4=99drzejewski-Szmek?= , Will Deacon , Alexander Viro , Eric Biederman , Kees Cook , Szabolcs Nagy , Mark Brown , Jeremy Linton , Topi Miettinen , "linux-mm@kvack.org" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-abi-devel@lists.sourceforge.net" References: <20220413134946.2732468-1-catalin.marinas@arm.com> <20220413134946.2732468-3-catalin.marinas@arm.com> <443d978a-7092-b5b1-22f3-ae3a997080ad@redhat.com> <59401856-0e45-0ee6-1e45-667c8e00cf21@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22.04.22 15:12, Catalin Marinas wrote: > On Fri, Apr 22, 2022 at 01:04:31PM +0200, David Hildenbrand wrote: >> On 22.04.22 12:28, Catalin Marinas wrote: >>> On Thu, Apr 21, 2022 at 06:37:49PM +0100, David Hildenbrand wrote: >>>> Note that in the (FOLL_WRITE|FOLL_FORCE) we only require VM_MAYWRITE on >>>> the vma and trigger a write fault. As the VMA is not VM_WRITE, we won't >>>> actually map the PTE writable, but set it dirty. GUP will retry, find a >>>> R/O pte that is dirty and where it knows that it broke COW and will >>>> allow the read access, although the PTE is R/O. >>>> >>>> That mechanism is required to e.g., set breakpoints in R/O MAP_PRIVATE >>>> kernel sections, but it's used elsewhere for page pinning as well. >>>> >>>> My gut feeling is that GUP(FOLL_WRITE|FOLL_FORCE) could be used right >>>> now to bypass that mechanism, I might be wrong. >>> >>> GUP can be used to bypass this. But if an attacker can trigger such GUP >>> paths via a syscall (e.g. ptrace(PTRACE_POKEDATA)), I think we need the >>> checks on those paths (and reject the syscall) rather than on >>> mmap/mprotect(). This would be covered by something like CAP_SYS_PTRACE. >> >> I was told that RDMA uses FOLL_FORCE|FOLL_WRITE and is available to >> unprivileged users. > > Ah, do they really need this? At a quick search, ib_umem_get() for > example: > > unsigned int gup_flags = FOLL_WRITE; > ... > if (!umem->writable) > gup_flags |= FOLL_FORCE; > > I guess with a new MDWE flag we can make the GUP code ignore FOLL_FORCE > if VM_EXEC. > It's, for example, required when you have a MAP_PRIVATE but PROT_READ mapping and want to take a reliable R/O (!) pin. Without FOLL_FORCE|FOLL_WRITE you'd be pinning a (shared zeropage, pagecache) page that will get replaced by an anonymous page in the COW handler, after mprotect(PROT_READ|PROT_WRITE) followed by a write access. That was an issue for RDMA in the past, that's why we have that handling in place IIRC. Yes, it's ugly. -- Thanks, David / dhildenb