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[86.156.84.164]) by smtp.gmail.com with ESMTPSA id v15-20020a05600c444f00b003f09cda253esm9304031wmn.34.2023.04.21.14.23.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 21 Apr 2023 14:23:13 -0700 (PDT) Date: Fri, 21 Apr 2023 22:23:12 +0100 From: Lorenzo Stoakes To: Jan Kara Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Matthew Wilcox , Mike Kravetz , Muchun Song , Alexander Viro , Christian Brauner , Andy Lutomirski Subject: Re: [RFC PATCH 0/3] permit write-sealed memfd read-only shared mappings Message-ID: References: <20230421090126.tmem27kfqamkdaxo@quack3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230421090126.tmem27kfqamkdaxo@quack3> X-Rspam-User: X-Rspamd-Queue-Id: 0087F10000D X-Rspamd-Server: rspam01 X-Stat-Signature: zqpiag7xbzxb619ny8xs1zoq519co3q4 X-HE-Tag: 1682112195-457687 X-HE-Meta: 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 zJ6/ttR4 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Apr 21, 2023 at 11:01:26AM +0200, Jan Kara wrote: > Hi! > > On Mon 03-04-23 23:28:29, Lorenzo Stoakes wrote: > > This patch series is in two parts:- > > > > 1. Currently there are a number of places in the kernel where we assume > > VM_SHARED implies that a mapping is writable. Let's be slightly less > > strict and relax this restriction in the case that VM_MAYWRITE is not > > set. > > > > This should have no noticeable impact as the lack of VM_MAYWRITE implies > > that the mapping can not be made writable via mprotect() or any other > > means. > > > > 2. Align the behaviour of F_SEAL_WRITE and F_SEAL_FUTURE_WRITE on mmap(). > > The latter already clears the VM_MAYWRITE flag for a sealed read-only > > mapping, we simply extend this to F_SEAL_WRITE too. > > > > For this to have effect, we must also invoke call_mmap() before > > mapping_map_writable(). > > > > As this is quite a fundamental change on the assumptions around VM_SHARED > > and since this causes a visible change to userland (in permitting read-only > > shared mappings on F_SEAL_WRITE mappings), I am putting forward as an RFC > > to see if there is anything terribly wrong with it. > > So what I miss in this series is what the motivation is. Is it that you need > to map F_SEAL_WRITE read-only? Why? > This originated from the discussion in [1], which refers to the bug reported in [2]. Essentially the user is write-sealing a memfd then trying to mmap it read-only, but receives an -EPERM error. F_SEAL_FUTURE_WRITE _does_ explicitly permit this but F_SEAL_WRITE does not. The fcntl() man page states: Furthermore, trying to create new shared, writable memory-mappings via mmap(2) will also fail with EPERM. So the kernel does not behave as the documentation states. I took the user-supplied repro and slightly modified it, enclosed below. After this patch series, this code works correctly. I think there's definitely a case for the VM_MAYWRITE part of this patch series even if the memfd bits are not considered useful, as we do seem to make the implicit assumption that MAP_SHARED == writable even if !VM_MAYWRITE which seems odd. Reproducer:- int main() { int fd = memfd_create("test", MFD_ALLOW_SEALING); if (fd == -1) { perror("memfd_create"); return EXIT_FAILURE; } write(fd, "test", 4); if (fcntl(fd, F_ADD_SEALS, F_SEAL_WRITE) == -1) { perror("fcntl"); return EXIT_FAILURE; } void *ret = mmap(NULL, 4, PROT_READ, MAP_SHARED, fd, 0); if (ret == MAP_FAILED) { perror("mmap"); return EXIT_FAILURE; } return EXIT_SUCCESS; } [1]:https://lore.kernel.org/all/20230324133646.16101dfa666f253c4715d965@linux-foundation.org/ [2]:https://bugzilla.kernel.org/show_bug.cgi?id=217238 > Honza > -- > Jan Kara > SUSE Labs, CR