From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 In-Reply-To: <20151210012130.GA17673@infradead.org> References: <20151209225148.GA14794@www.outflux.net> <20151210012130.GA17673@infradead.org> Date: Wed, 9 Dec 2015 19:25:26 -0800 Message-ID: Subject: Re: [PATCH v5] fs: clear file privilege bits when mmap writing From: Kees Cook To: Christoph Hellwig Cc: Andrew Morton , Jan Kara , yalin wang , Willy Tarreau , "Eric W. Biederman" , Alexander Viro , "linux-fsdevel@vger.kernel.org" , Linux-MM , LKML Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: On Wed, Dec 9, 2015 at 5:21 PM, Christoph Hellwig wrote: >> 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). We >> could do this during vm_mmap_pgoff, but that would need coverage in >> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem >> again. We could clear at open() time, but it's possible things are >> accidentally opening with O_RDWR and only reading. Better to clear on >> close and error failures (i.e. an improvement over now, which is not >> clearing at all). >> >> Instead, detect the need to clear the bits during the page fault, and >> actually remove the bits during final fput. Since the file was open for >> writing, it wouldn't have been possible to execute it yet. > > >> >> Signed-off-by: Kees Cook >> --- >> I think this is the best we can do; everything else is blocked by mmap_sem. > > It should be done at mmap time, before even taking mmap_sem. > > Adding a new field for this to strut file isn't really acceptable. I already covered this: there's no way to handle the mprotect case -- checking for MAP_SHARED is under mmap_sem still. -Kees -- 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