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[209.132.180.67]) by mx.google.com with ESMTP id p124si632904oig.195.2019.12.16.10.21.05; Mon, 16 Dec 2019 10:21:05 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of stable-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=default header.b=NlbFjfDU; spf=pass (google.com: best guess record for domain of stable-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=stable-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732192AbfLPSVE (ORCPT + 14 others); Mon, 16 Dec 2019 13:21:04 -0500 Received: from mail.kernel.org ([198.145.29.99]:53516 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732183AbfLPSVE (ORCPT ); Mon, 16 Dec 2019 13:21:04 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id AD5252166E; Mon, 16 Dec 2019 18:21:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576520463; bh=Uv07aAKk39n58UabQhF6Iu9klPWARZ2sCElrDYxIY9k=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=NlbFjfDUSI/z1+yS/mjcv0oZIoKMS800oD7IJA7L8zniNR8dRk79gK5C4wAZMF8xB V2xaTkxNtgDT6R5uAO2aAZ11k5t7T1aQoLullll2EMQw+uIt4bm+o0g6f7O9dIwz2Z NjgRhSxVyklgPNt3zo+tZbAzWQgncqgtgw4rikUs= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Nicolas Geoffray , "Joel Fernandes (Google)" , Hugh Dickins , Shuah Khan , Andrew Morton , Linus Torvalds Subject: [PATCH 5.4 164/177] mm, memfd: fix COW issue on MAP_PRIVATE and F_SEAL_FUTURE_WRITE mappings Date: Mon, 16 Dec 2019 18:50:20 +0100 Message-Id: <20191216174849.586463116@linuxfoundation.org> X-Mailer: git-send-email 2.24.1 In-Reply-To: <20191216174811.158424118@linuxfoundation.org> References: <20191216174811.158424118@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Nicolas Geoffray commit 05d351102dbe4e103d6bdac18b1122cd3cd04925 upstream. F_SEAL_FUTURE_WRITE has unexpected behavior when used with MAP_PRIVATE: A private mapping created after the memfd file that gets sealed with F_SEAL_FUTURE_WRITE loses the copy-on-write at fork behavior, meaning children and parent share the same memory, even though the mapping is private. The reason for this is due to the code below: static int shmem_mmap(struct file *file, struct vm_area_struct *vma) { struct shmem_inode_info *info = SHMEM_I(file_inode(file)); if (info->seals & F_SEAL_FUTURE_WRITE) { /* * New PROT_WRITE and MAP_SHARED mmaps are not allowed when * "future write" seal active. */ if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE)) return -EPERM; /* * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED * read-only mapping, take care to not allow mprotect to revert * protections. */ vma->vm_flags &= ~(VM_MAYWRITE); } ... } And for the mm to know if a mapping is copy-on-write: static inline bool is_cow_mapping(vm_flags_t flags) { return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE; } The patch fixes the issue by making the mprotect revert protection happen only for shared mappings. For private mappings, using mprotect will have no effect on the seal behavior. The F_SEAL_FUTURE_WRITE feature was introduced in v5.1 so v5.3.x stable kernels would need a backport. [akpm@linux-foundation.org: reflow comment, per Christoph] Link: http://lkml.kernel.org/r/20191107195355.80608-1-joel@joelfernandes.org Fixes: ab3948f58ff84 ("mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd") Signed-off-by: Nicolas Geoffray Signed-off-by: Joel Fernandes (Google) Cc: Hugh Dickins Cc: Shuah Khan Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/shmem.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) --- a/mm/shmem.c +++ b/mm/shmem.c @@ -2213,11 +2213,14 @@ static int shmem_mmap(struct file *file, return -EPERM; /* - * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED - * read-only mapping, take care to not allow mprotect to revert - * protections. + * Since an F_SEAL_FUTURE_WRITE sealed memfd can be mapped as + * MAP_SHARED and read-only, take care to not allow mprotect to + * revert protections on such mappings. Do this only for shared + * mappings. For private mappings, don't need to mask + * VM_MAYWRITE as we still want them to be COW-writable. */ - vma->vm_flags &= ~(VM_MAYWRITE); + if (vma->vm_flags & VM_SHARED) + vma->vm_flags &= ~(VM_MAYWRITE); } file_accessed(file);