From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f177.google.com (mail-ig0-f177.google.com [209.85.213.177]) by kanga.kvack.org (Postfix) with ESMTP id D9ABA6B027A for ; Mon, 7 Dec 2015 19:40:15 -0500 (EST) Received: by igl9 with SMTP id 9so86096649igl.0 for ; Mon, 07 Dec 2015 16:40:15 -0800 (PST) Received: from mail-ig0-x234.google.com (mail-ig0-x234.google.com. [2607:f8b0:4001:c05::234]) by mx.google.com with ESMTPS id i86si1515267ioo.118.2015.12.07.16.40.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 07 Dec 2015 16:40:15 -0800 (PST) Received: by igcmv3 with SMTP id mv3so91012434igc.0 for ; Mon, 07 Dec 2015 16:40:15 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: <20151203000342.GA30015@www.outflux.net> Date: Mon, 7 Dec 2015 16:40:14 -0800 Message-ID: Subject: Re: [PATCH v2] clear file privilege bits when mmap writing From: Kees Cook Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: 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 On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook wrote: > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang wro= te: >> >>> 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 >>> =E2=80=94 >> >> 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 --=20 Kees Cook Chrome OS & Brillo Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org