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

From: MickaÃl SalaÃn
Date: Mon Sep 09 2019 - 05:33:51 EST

On 06/09/2019 22:06, Andy Lutomirski wrote:
>> On Sep 6, 2019, at 12:43 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>>> On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
>>>> On 2019-09-06, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>>>>> On Fri, 2019-09-06 at 18:06 +0200, MickaÃl SalaÃn 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.

This is the purpose of this patch series. It doesn't make sense to add
memory restrictions nor constrain fexecve and such.

> - Make the fd fexecve-able.
> - Make the resulting fd mappable PROT_EXEC.
> - Make the resulting fd upgradable.
> Iâm not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO.

MickaÃl SalaÃn

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