> On Sep 9, 2019, at 2:18 AM, Mickaël Salaün <mickael.salaun@xxxxxxxxxxx> wrote: > > >> On 06/09/2019 20:41, Andy Lutomirski wrote: >> >> >>>> On Sep 6, 2019, at 11:38 AM, Jeff Layton <jlayton@xxxxxxxxxx> 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@xxxxxxxxxxxxx/ >>>> >>>> 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. > > The sysctl is required to enable the adoption of this flag without > breaking existing systems. Current systems may have "noexec" on mount > points containing scripts. Without giving the ability to the sysadmin to > control that behavior, updating to a newer version of an interpreter > using O_MAYEXEC may break such systems. > > How would you do this with ld.so.conf ? > By telling user code not to use O_MAYEXEC? Alternatively, you could allow O_MAYEXEC even on a noexec mount and have a strong_noexec option that blocks it.
