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Mon, 16 Oct 2023 09:27:05 -0700 (PDT) Date: Mon, 16 Oct 2023 17:27:04 +0100 From: Lorenzo Stoakes To: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton Cc: Mike Kravetz , Muchun Song , Alexander Viro , Christian Brauner , Matthew Wilcox , Hugh Dickins , Andy Lutomirski , Jan Kara , linux-fsdevel@vger.kernel.org, bpf@vger.kernel.org, Naresh Kamboju , lkft-triage@lists.linaro.org Subject: Re: [PATCH v4 3/3] mm: perform the mapping_map_writable() check after call_mmap() Message-ID: References: <55e413d20678a1bb4c7cce889062bbb07b0df892.1697116581.git.lstoakes@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55e413d20678a1bb4c7cce889062bbb07b0df892.1697116581.git.lstoakes@gmail.com> X-Rspamd-Queue-Id: A1B49A0021 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: 4if59jbnfngq48bewpyp1ca8e9jjpheg X-HE-Tag: 1697473628-925699 X-HE-Meta: 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 +xFamaxT 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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 Thu, Oct 12, 2023 at 06:04:30PM +0100, Lorenzo Stoakes wrote: > In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to > clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap() > handler to do so. We would otherwise fail the mapping_map_writable() check > before we had the opportunity to avoid it. > > This patch moves this check after the call_mmap() invocation. Only memfd > actively denies write access causing a potential failure here (in > memfd_add_seals()), so there should be no impact on non-memfd cases. > > This patch makes the userland-visible change that MAP_SHARED, PROT_READ > mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed. > > There is a delicate situation with cleanup paths assuming that a writable > mapping must have occurred in circumstances where it may now not have. In > order to ensure we do not accidentally mark a writable file unwritable by > mistake, we explicitly track whether we have a writable mapping and > unmap only if we do. > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238 > Signed-off-by: Lorenzo Stoakes > --- > mm/mmap.c | 23 ++++++++++++++--------- > 1 file changed, 14 insertions(+), 9 deletions(-) > [snip] Andrew, could you apply the following -fix patch to this? As a bug was detected in the implementation [0] - I was being over-zealous in setting the writable_file_mapping flag and had falsely assumed vma->vm_file == file in all instances of the cleanup. The fix is to only set it in one place. [0]: https://lore.kernel.org/all/CA+G9fYtL7wK-dE-Tnz4t-GWmQb50EPYa=TWGjpgYU2Z=oeAO_w@mail.gmail.com/ ----8<---- >From 7feea6faada5b10a872c24755cc630220cba619a Mon Sep 17 00:00:00 2001 From: Lorenzo Stoakes Date: Mon, 16 Oct 2023 17:17:13 +0100 Subject: [PATCH] mm: perform the mapping_map_writable() check after call_mmap() Do not set writable_file_mapping in an instance where it is not appropriate to do so. Signed-off-by: Lorenzo Stoakes --- mm/mmap.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 7f45a08e7973..8b57e42fd980 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2923,10 +2923,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr, mm->map_count++; if (vma->vm_file) { i_mmap_lock_write(vma->vm_file->f_mapping); - if (vma_is_shared_maywrite(vma)) { + if (vma_is_shared_maywrite(vma)) mapping_allow_writable(vma->vm_file->f_mapping); - writable_file_mapping = true; - } flush_dcache_mmap_lock(vma->vm_file->f_mapping); vma_interval_tree_insert(vma, &vma->vm_file->f_mapping->i_mmap); -- 2.42.0