Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757328AbbLCAEj (ORCPT ); Wed, 2 Dec 2015 19:04:39 -0500 Received: from smtp.outflux.net ([198.145.64.163]:53217 "EHLO smtp.outflux.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754181AbbLCAEi (ORCPT ); Wed, 2 Dec 2015 19:04:38 -0500 Date: Wed, 2 Dec 2015 16:03:42 -0800 From: Kees Cook To: Andrew Morton Cc: Jan Kara , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v2] fs: clear file privilege bits when mmap writing Message-ID: <20151203000342.GA30015@www.outflux.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-HELO: www.outflux.net Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1753 Lines: 54 Normally, when a user can modify a file that has setuid or setgid bits, those bits are cleared when they are not the file owner or a member of the group. This is enforced when using write and truncate but not when writing to a shared mmap on the file. This could allow the file writer to gain privileges by changing a binary without losing the setuid/setgid/caps bits. Changing the bits requires holding inode->i_mutex, so it cannot be done during the page fault (due to mmap_sem being held during the fault). Instead, clear the bits if PROT_WRITE is being used at mmap time. Signed-off-by: Kees Cook Cc: stable@vger.kernel.org --- v2: - move check from page fault to mmap open --- mm/mmap.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/mm/mmap.c b/mm/mmap.c index 2ce04a649f6b..a27735aabc73 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1340,6 +1340,17 @@ unsigned long do_mmap(struct file *file, unsigned long addr, if (locks_verify_locked(file)) return -EAGAIN; + /* + * If we must remove privs, we do it here since + * doing it during page COW is expensive and + * cannot hold inode->i_mutex. + */ + if (prot & PROT_WRITE && !IS_NOSEC(inode)) { + mutex_lock(&inode->i_mutex); + file_remove_privs(file); + mutex_unlock(&inode->i_mutex); + } + vm_flags |= VM_SHARED | VM_MAYSHARE; if (!(file->f_mode & FMODE_WRITE)) vm_flags &= ~(VM_MAYWRITE | VM_SHARED); -- 1.9.1 -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/