Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756166AbbLHAkU (ORCPT ); Mon, 7 Dec 2015 19:40:20 -0500 Received: from mail-ig0-f177.google.com ([209.85.213.177]:35642 "EHLO mail-ig0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756001AbbLHAkP convert rfc822-to-8bit (ORCPT ); Mon, 7 Dec 2015 19:40:15 -0500 MIME-Version: 1.0 In-Reply-To: References: <20151203000342.GA30015@www.outflux.net> Date: Mon, 7 Dec 2015 16:40:14 -0800 X-Google-Sender-Auth: X2X_U6KQqUTwsMQTIm_QpZ3kpJU Message-ID: Subject: Re: [PATCH v2] clear file privilege bits when mmap writing From: Kees Cook To: yalin wang Cc: Andrew Morton , Jan Kara , Willy Tarreau , "Eric W. Biederman" , "Kirill A. Shutemov" , Oleg Nesterov , Rik van Riel , Chen Gang , Davidlohr Bueso , Andrea Arcangeli , Linux-MM , LKML Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1828 Lines: 47 On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook wrote: > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang wrote: >> >>> On Dec 2, 2015, at 16:03, Kees Cook wrote: >>> >>> 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 >>> — >> >> is this means mprotect() sys call also need add this check? >> mprotect() can change to PROT_WRITE, then it can write to a >> read only map again , also a secure hole here . > > Yes, good point. This needs to be added. I will send a new patch. Thanks! This continues to look worse and worse. So... to check this at mprotect time, I have to know it's MAP_SHARED, but that's in the vma_flags, which I can only see after holding mmap_sem. The best I can think of now is to strip the bits at munmap time, since you can't execute an mmapped file until it closes. Jan, thoughts on this? -Kees -- 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/