From patchwork Fri Oct 5 19:27:27 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Joel Fernandes X-Patchwork-Id: 10628683 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 73856112B for ; Fri, 5 Oct 2018 19:27:43 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 60A0A29BA8 for ; Fri, 5 Oct 2018 19:27:43 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 5446E29C2B; Fri, 5 Oct 2018 19:27:43 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.0 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id BE9A429BA8 for ; Fri, 5 Oct 2018 19:27:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728170AbeJFC1u (ORCPT ); Fri, 5 Oct 2018 22:27:50 -0400 Received: from mail-pf1-f196.google.com ([209.85.210.196]:41245 "EHLO mail-pf1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728042AbeJFC1t (ORCPT ); Fri, 5 Oct 2018 22:27:49 -0400 Received: by mail-pf1-f196.google.com with SMTP id m77-v6so5513942pfi.8 for ; Fri, 05 Oct 2018 12:27:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=O8MUoMdyZp7VnFk86cz/bxEv7KcxO90a6K0fUhGYRKA=; b=TyGED5pyaQZWWBdVUycfsGP3LhusgFUJp+EHbH6ZUBs8OYQhY+LDyCi36QgONUT/OE epxlgl1k5wW3Sqr4sf1RhIbzjF21B3QsuLSet6ognNUqOWjcKtrN2+YwY0U6H915YVLc OLMH6fniWBFsmSzTcdCGtblCqe+LqgcccbJ00= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=O8MUoMdyZp7VnFk86cz/bxEv7KcxO90a6K0fUhGYRKA=; b=LTbcZC7KWtNL1AKePnqG4MKv0WLi8UaQV3y2qL05vrdVJHWf6EF0t6bSsws2Cy098Q Egz/PIjMYs9S/tPHpr1+QRHN14tKfl17aW5/LuTz/lJbL14T4Ska0NO7Vd+wg6Omd841 FNCtKzsQpyMDgQP9Vfdh3UZdih5mXF4kXiXPEt51QDvKtP4gn3kfR6syr7ZfiqYaw8oW tkns1ZBsfnxWWRo9lb897ZoyoMPyptuCtxqDl3YI7wTcsqZjTvDE4WF/FYWeQmBbWGek qbXx6shLsuodZj2QdgfCurC+yxFkEu+KLCQFaFswjPUlJZ/V/VKUgglkzDhHMCFJXoVb yDow== X-Gm-Message-State: ABuFfoi/TpgJKkOwJ0pu2YgBET+TJkcGwtvGF2lCzjm+feMPzEAK3sF9 EojLR8RIYNCoemufRC5FenZv3g== X-Google-Smtp-Source: ACcGV637nbvtU1/IOfHQBe0ZOxC4LT6wSr5iQYENxkPw88+Kap9FfbHNNYbfx2Q/1MyJP5ytiF079A== X-Received: by 2002:a62:968a:: with SMTP id s10-v6mr13202004pfk.191.1538767660352; Fri, 05 Oct 2018 12:27:40 -0700 (PDT) Received: from joelaf.mtv.corp.google.com ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id z15-v6sm11469300pfn.170.2018.10.05.12.27.38 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 05 Oct 2018 12:27:38 -0700 (PDT) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org Cc: kernel-team@android.com, "Joel Fernandes (Google)" , jreck@google.com, john.stultz@linaro.org, tkjos@google.com, gregkh@linuxfoundation.org, Al Viro , Andrew Morton , "J. Bruce Fields" , Jeff Layton , Khalid Aziz , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Mike Kravetz Subject: [PATCH RFC] mm: Add an fs-write seal to memfd Date: Fri, 5 Oct 2018 12:27:27 -0700 Message-Id: <20181005192727.167933-1-joel@joelfernandes.org> X-Mailer: git-send-email 2.19.0.605.g01d371f741-goog MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP Android uses ashmem for sharing memory regions. We are looking forward to migrating all usecases of ashmem to memfd so that we can possibly remove the ashmem driver in the future from staging while also benefiting from using memfd and contributing to it. Note staging drivers are also not ABI and generally can be removed at anytime. One of the main usecases Android has is the ability to create a region and mmap it as writeable, then drop its protection for "future" writes while keeping the existing already mmap'ed writeable-region active. This allows us to implement a usecase where receivers of the shared memory buffer can get a read-only view, while the sender continues to write to the buffer. See CursorWindow in Android for more details: https://developer.android.com/reference/android/database/CursorWindow This usecase cannot be implemented with the existing F_SEAL_WRITE seal. To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which prevents any future mmap and write syscalls from succeeding while keeping the existing mmap active. The following program shows the seal working in action: int main() { int ret, fd; void *addr, *addr2, *addr3, *addr1; ret = memfd_create_region("test_region", REGION_SIZE); printf("ret=%d\n", ret); fd = ret; // Create map addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (addr == MAP_FAILED) printf("map 0 failed\n"); else printf("map 0 passed\n"); if ((ret = write(fd, "test", 4)) != 4) printf("write failed even though no fs-write seal " "(ret=%d errno =%d)\n", ret, errno); else printf("write passed\n"); addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (addr1 == MAP_FAILED) perror("map 1 prot-write failed even though no seal\n"); else printf("map 1 prot-write passed as expected\n"); ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FS_WRITE); if (ret == -1) printf("fcntl failed, errno: %d\n", errno); else printf("fs-write seal now active\n"); if ((ret = write(fd, "test", 4)) != 4) printf("write failed as expected due to fs-write seal\n"); else printf("write passed (unexpected)\n"); addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); if (addr2 == MAP_FAILED) perror("map 2 prot-write failed as expected due to seal\n"); else printf("map 2 passed\n"); addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0); if (addr3 == MAP_FAILED) perror("map 3 failed\n"); else printf("map 3 prot-read passed as expected\n"); } The output of running this program is as follows: ret=3 map 0 passed write passed map 1 prot-write passed as expected fs-write seal now active write failed as expected due to fs-write seal map 2 prot-write failed as expected due to seal : Permission denied map 3 prot-read passed as expected Cc: jreck@google.com Cc: john.stultz@linaro.org Cc: tkjos@google.com Cc: gregkh@linuxfoundation.org Signed-off-by: Joel Fernandes (Google) --- include/uapi/linux/fcntl.h | 1 + mm/memfd.c | 6 +++++- 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h index 6448cdd9a350..bafd1233b6a8 100644 --- a/include/uapi/linux/fcntl.h +++ b/include/uapi/linux/fcntl.h @@ -41,6 +41,7 @@ #define F_SEAL_SHRINK 0x0002 /* prevent file from shrinking */ #define F_SEAL_GROW 0x0004 /* prevent file from growing */ #define F_SEAL_WRITE 0x0008 /* prevent writes */ +#define F_SEAL_FS_WRITE 0x0010 /* prevent all write-related syscalls */ /* (1U << 31) is reserved for signed error codes */ /* diff --git a/mm/memfd.c b/mm/memfd.c index 2bb5e257080e..4b09f446c2d2 100644 --- a/mm/memfd.c +++ b/mm/memfd.c @@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file) #define F_ALL_SEALS (F_SEAL_SEAL | \ F_SEAL_SHRINK | \ F_SEAL_GROW | \ - F_SEAL_WRITE) + F_SEAL_WRITE | \ + F_SEAL_FS_WRITE) static int memfd_add_seals(struct file *file, unsigned int seals) { @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals) } } + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE)) + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE); + *file_seals |= seals; error = 0;