Date: Fri, 6 Sep 2019 11:41:06 -0700 From: Andy Lutomirski <luto@...capital.net> To: Jeff Layton <jlayton@...nel.org> Cc: Mickaël Salaün <mickael.salaun@....gouv.fr>, Florian Weimer <fweimer@...hat.com>, Mickaël Salaün <mic@...ikod.net>, linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>, 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@...ts.openwall.com, linux-api@...r.kernel.org, linux-security-module@...r.kernel.org, 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 11:38 AM, Jeff Layton <jlayton@...nel.org> wrote: > >> On Fri, 2019-09-06 at 19:14 +0200, Mickaël Salaün wrote: >>> On 06/09/2019 18:48, Jeff Layton wrote: >>>> On Fri, 2019-09-06 at 18:06 +0200, Mickaël Salaün wrote: >>>>> On 06/09/2019 17:56, Florian Weimer wrote: >>>>> Let's assume I want to add support for this to the glibc dynamic loader, >>>>> while still being able to run on older kernels. >>>>> >>>>> Is it safe to try the open call first, with O_MAYEXEC, and if that fails >>>>> with EINVAL, try again without O_MAYEXEC? >>>> >>>> The kernel ignore unknown open(2) flags, so yes, it is safe even for >>>> older kernel to use O_MAYEXEC. >>>> >>> >>> Well...maybe. What about existing programs that are sending down bogus >>> open flags? Once you turn this on, they may break...or provide a way to >>> circumvent the protections this gives. >> >> Well, I don't think we should nor could care about bogus programs that >> do not conform to the Linux ABI. >> > > But they do conform. The ABI is just undefined here. Unknown flags are > ignored so we never really know if $random_program may be setting them. > >>> Maybe this should be a new flag that is only usable in the new openat2() >>> syscall that's still under discussion? That syscall will enforce that >>> all flags are recognized. You presumably wouldn't need the sysctl if you >>> went that route too. >> >> Here is a thread about a new syscall: >> https://email@example.com/ >> >> I don't think it fit well with auditing nor integrity. Moreover using >> the current open(2) behavior of ignoring unknown flags fit well with the >> usage of O_MAYEXEC (because it is only a hint to the kernel about the >> use of the *opened* file). >> > > The fact that open and openat didn't vet unknown flags is really a bug. > > Too late to fix it now, of course, and as Aleksa points out, we've > worked around that in the past. Now though, we have a new openat2 > syscall on the horizon. There's little need to continue these sorts of > hacks. > > New open flags really have no place in the old syscalls, IMO. > >>> Anyone that wants to use this will have to recompile anyway. If the >>> kernel doesn't support openat2 or if the flag is rejected then you know >>> that you have no O_MAYEXEC support and can decide what to do. >> >> If we want to enforce a security policy, we need to either be the system >> administrator or the distro developer. If a distro ship interpreters >> using this flag, we don't need to recompile anything, but we need to be >> able to control the enforcement according to the mount point >> configuration (or an advanced MAC, or an IMA config). I don't see why an >> userspace process should check if this flag is supported or not, it >> should simply use it, and the sysadmin will enable an enforcement if it >> makes sense for the whole system. >> > > A userland program may need to do other risk mitigation if it sets > O_MAYEXEC and the kernel doesn't recognize it. > > Personally, here's what I'd suggest: > > - Base this on top of the openat2 set > - Change it that so that openat2() files are non-executable by default. Anyone wanting to do that needs to set O_MAYEXEC or upgrade the fd somehow. > - Only have the openat2 syscall pay attention to O_MAYEXEC. Let open and openat continue ignoring the new flag. > > That works around a whole pile of potential ABI headaches. Note that > we'd need to make that decision before the openat2 patches are merged. > > Even better would be to declare the new flag in some openat2-only flag > space, so there's no confusion about it being supported by legacy open > calls. > > If glibc wants to implement an open -> openat2 wrapper in userland > later, it can set that flag in the wrapper implicitly to emulate the old > behavior. > > Given that you're going to have to recompile software to take advantage > of this anyway, what's the benefit to changing legacy syscalls? > >>>>> Or do I risk disabling this security feature if I do that? >>>> >>>> It is only a security feature if the kernel support it, otherwise it is >>>> a no-op. >>>> >>> >>> With a security feature, I think we really want userland to aware of >>> whether it works. >> >> If userland would like to enforce something, it can already do it >> without any kernel modification. The goal of the O_MAYEXEC flag is to >> enable the kernel, hence sysadmins or system designers, to enforce a >> global security policy that makes sense. >> > > I don't see how this helps anything if you can't tell whether the kernel > recognizes the damned thing. Also, our track record with global sysctl > switches like this is pretty poor. They're an administrative headache as > well as a potential attack vector. I tend to agree. The sysctl seems like it’s asking for trouble. I can see an ld.so.conf option to turn this thing off making sense.
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