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

Andy Lutomirski
2019-09-06T20:06:24Z

> On Sep 6, 2019, at 12:43 PM, Jeff Layton <jlayton@kernel.org> wrote:
>
>> On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
>>> On 2019-09-06, Jeff Layton <jlayton@kernel.org> 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.
>>
>> It should be noted that this has been a valid concern for every new O_*
>> flag introduced (and yet we still introduced new flags, despite the
>> concern) -- though to be fair, O_TMPFILE actually does have a
>> work-around with the O_DIRECTORY mask setup.
>>
>> The openat2() set adds O_EMPTYPATH -- though in fairness it's also
>> backwards compatible because empty path strings have always given ENOENT
>> (or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings.
>>
>>> 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.
>>
>> I'm also interested in whether we could add an UPGRADE_NOEXEC flag to
>> how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit for
>> it, since I'd heard about this work through the grape-vine).
>>
>
> I rather like the idea of having openat2 fds be non-executable by
> default, and having userland request it specifically via O_MAYEXEC (or
> some similar openat2 flag) if it's needed. Then you could add an
> UPGRADE_EXEC flag instead?
>
> That seems like something reasonable to do with a brand new API, and
> might be very helpful for preventing certain classes of attacks.
>
>

There are at least four concepts of executability here:

- Just check the file mode and any other relevant permissions. Return a normal fd. Makes sense for script interpreters, perhaps.

- Make the fd fexecve-able.

- Make the resulting fd mappable PROT_EXEC.

- Make the resulting fd upgradable.

I’m not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO.