Date: Fri, 6 Sep 2019 14:27:19 -0700 From: Andy Lutomirski <luto@...capital.net> To: Jeff Layton <jlayton@...nel.org> Cc: Aleksa Sarai <cyphar@...har.com>, Mickaël Salaün <mickael.salaun@....gouv.fr>, Florian Weimer <fweimer@...hat.com>, Mickaël Salaün <mic@...ikod.net>, LKML <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>, Christian Heimes <christian@...hon.org>, Daniel Borkmann <daniel@...earbox.net>, Eric Chiang <ericchiang@...gle.com>, James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>, Michael Kerrisk <mtk.manpages@...il.com>, Mimi Zohar <zohar@...ux.ibm.com>, Philippe Trébuchet <philippe.trebuchet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>, Sean Christopherson <sean.j.christopherson@...el.com>, Shuah Khan <shuah@...nel.org>, Song Liu <songliubraving@...com>, Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>, Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, LSM List <linux-security-module@...r.kernel.org>, Linux FS Devel <linux-fsdevel@...r.kernel.org> Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() > On Sep 6, 2019, at 1:51 PM, Jeff Layton <jlayton@...nel.org> wrote: > > On Fri, 2019-09-06 at 13:06 -0700, Andy Lutomirski wrote: > >> I’m not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO. > > Good point. Upgradability is definitely orthogonal, though the idea > there is to alter the default behavior. If the default is NOEXEC then > UPGRADE_EXEC would make sense. > > In any case, I was mostly thinking about the middle two in your list > above. After more careful reading of the patches, I now get get that > Mickaël is more interested in the first, and that's really a different > sort of use-case. > > Most opens never result in the fd being fed to fexecve or mmapped with > PROT_EXEC, so having userland explicitly opt-in to allowing that during > the open sounds like a reasonable thing to do. > > But I get that preventing execution via script interpreters of files > that are not executable might be something nice to have. > > Perhaps we need two flags for openat2? > > OA2_MAYEXEC : test that permissions allow execution and that the file > doesn't reside on a noexec mount before allowing the open > > OA2_EXECABLE : only allow fexecve or mmapping with PROT_EXEC if the fd > was opened with this > > > We could go one step farther and have three masks: check_perms, fd_perms, and upgrade_perms. check_perms says “fail if I don’t have these perms”. fd_perms is the permissions on the returned fd, and upgrade_perms is the upgrade mask. (fd_perms & ~check_perms) != 0 is an error. This makes it possible to say "I want to make sure the file is writable, but I don't actually want to write to it", which could plausibly be useful. I would argue that these things should have new, sane bits, e.g. FILE_READ, FILE_WRITE, and FILE_EXECUTE (or maybe FILE_MAP_EXEC and FILE_EXECVE). And maybe there should be at least 16 bits for each mask reserved. Windows has a lot more mode bits than Linux, and it's not entirely nuts. We do *not* need any direct equivalent of O_RDWR for openat2(). --Andy
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.