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[2003:cb:c702:de00:711b:76af:b335:9b70]) by smtp.gmail.com with ESMTPSA id l14-20020adffe8e000000b00207af9cdd90sm2618386wrr.39.2022.04.21.10.37.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Apr 2022 10:37:50 -0700 (PDT) Message-ID: <443d978a-7092-b5b1-22f3-ae3a997080ad@redhat.com> Date: Thu, 21 Apr 2022 19:37:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Content-Language: en-US To: Catalin Marinas , Andrew Morton , Christoph Hellwig , Lennart Poettering , =?UTF-8?Q?Zbigniew_J=c4=99drzejewski-Szmek?= Cc: 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> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag In-Reply-To: <20220413134946.2732468-3-catalin.marinas@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13.04.22 15:49, Catalin Marinas wrote: > The aim of such policy is to prevent a user task from inadvertently > creating an executable mapping that is or was writeable (and > subsequently made read-only). > > An example of mmap() returning -EACCESS if the policy is enabled: > > mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, flags, 0, 0); > > Similarly, mprotect() would return -EACCESS below: > > addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0); > mprotect(addr, size, PROT_READ | PROT_WRITE | PROT_EXEC); > > With the past vma writeable permission tracking, mprotect() below would > also fail with -EACCESS: > > addr = mmap(0, size, PROT_READ | PROT_WRITE, flags, 0, 0); > mprotect(addr, size, PROT_READ | PROT_EXEC); > > While the above could be achieved by checking PROT_WRITE & PROT_EXEC on > mmap/mprotect and denying mprotect(PROT_EXEC) altogether (current > systemd MDWE approach via SECCOMP BPF filters), we want the following > scenario to succeed: > > addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0); > mprotect(addr, size, PROT_READ | PROT_EXEC | PROT_BTI); > > where PROT_BTI enables branch tracking identification on arm64. > > The choice for a DENY_WRITE_EXEC personality flag, inherited on fork() > and execve(), was made by analogy to READ_IMPLIES_EXEC. > > Note that it is sufficient to check for VM_WAS_WRITE in > map_deny_write_exec() as this flag is always set on VM_WRITE mappings. > > Signed-off-by: Catalin Marinas > Cc: Christoph Hellwig > Cc: Andrew Morton How does this interact with get_user_pages(FOLL_WRITE|FOLL_FORCE) on a VMA that is VM_MAYWRITE but not VM_WRITE? Is it handled accordingly? 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. -- Thanks, David / dhildenb