From patchwork Tue Oct 9 22:20:41 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Joel Fernandes X-Patchwork-Id: 10633323 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 2376B46E4 for ; Tue, 9 Oct 2018 22:21:07 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 141A329B8B for ; Tue, 9 Oct 2018 22:21:07 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 11AD529B8C; Tue, 9 Oct 2018 22:21:07 +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=unavailable 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 813CA29B94 for ; Tue, 9 Oct 2018 22:21:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727989AbeJJFkK (ORCPT ); Wed, 10 Oct 2018 01:40:10 -0400 Received: from mail-pg1-f195.google.com ([209.85.215.195]:42779 "EHLO mail-pg1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727972AbeJJFkK (ORCPT ); Wed, 10 Oct 2018 01:40:10 -0400 Received: by mail-pg1-f195.google.com with SMTP id i4-v6so1493183pgq.9 for ; Tue, 09 Oct 2018 15:21:04 -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=zE/nAc/JVnWc4CMHOs76nC5pBisapw03s96G8eS2xeQ=; b=DqgZgnQzBHyIneOhD1h4aX7Xyu4gvvYOv+Vk54QCfHuLo0vCFlRbhsT9KfXoGwGe20 pD0AS4uwoDw40u3H6LbfVxJ9WcCPTcJck4wiCPzTNxQ8eyB/POuCYpno94TiBdgJ6cuq N2SId8NAkSqk3T7Td+BFsEbrFN7Q8PFvGvKdk= 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=zE/nAc/JVnWc4CMHOs76nC5pBisapw03s96G8eS2xeQ=; b=hxk+iApgxTtFCA3Ghgt0pJ2AfXj2shAIWcWxxNHdXgnu2hicQTOrXLlXc9iwfVJjgu 6rO4/z0hjlp4AqwqdfYWwsZA3W1Fm6JMf4zwDkaCvNfCgDDSfJu9cJB3xJ9cyXrGGfda 24h9TLGhq0/wQB4/uDdrYMU1xDOGQtQ35eIE0eeyBVVQIeXFqBVgb8tgizxZEa7nvqMI IFys4edb0glCrCbQ0o7Uv9xCTD3L1oHyzv71yPMclhLUV2HQN6eaIZ1Tn0LpZ2etWdhU QZMXSFToqAchp4UKj/cWlftfq1S9RH327wZhG3klOKJ2soDbGC3uQV7pFF0Q8skmpqQk vPnw== X-Gm-Message-State: ABuFfoiQWDQq0wyCFjA2m7v2SrA6EWe/svJPoD+YxzfSku3wAMrQl+BG pLprh0lM5mwexl7eV6E2Sm99BA== X-Google-Smtp-Source: ACcGV61RXAN2jDLnDPfW2F40XM1mwAwHFEHxP3Mu2de0YVywOd11qePJX02C+LZ4ZjNTcmH4lZpKdQ== X-Received: by 2002:a62:8891:: with SMTP id l139-v6mr31867512pfd.198.1539123663946; Tue, 09 Oct 2018 15:21:03 -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-v6sm26900856pfn.170.2018.10.09.15.21.02 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 09 Oct 2018 15:21:02 -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, Andrew Morton , dancol@google.com, "J. Bruce Fields" , Jeff Layton , Khalid Aziz , linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Mike Kravetz , minchan@google.com, Shuah Khan Subject: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd Date: Tue, 9 Oct 2018 15:20:41 -0700 Message-Id: <20181009222042.9781-1-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 Note: This seal will also prevent growing and shrinking of the memfd. This is not something we do in Android so it does not affect us, however I have mentioned this behavior of the seal in the manpage. Cc: jreck@google.com Cc: john.stultz@linaro.org Cc: tkjos@google.com Cc: gregkh@linuxfoundation.org Signed-off-by: Joel Fernandes (Google) Reviewed-by: John Stultz --- v1->v2: No change, just added selftests to the series. manpages are ready and I'll submit them once the patches are accepted. 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 c98312fa78a5..fe44a2035edf 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 27069518e3c5..9b8855b80de9 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;