[PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

[Andy Lutomirski]

> On Sep 6, 2019, at 11:38 AM, Jeff Layton <jlayton at kernel.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://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.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.