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Date: Fri, 6 Sep 2019 13:06:00 -0700
From: Andy Lutomirski <>
To: Jeff Layton <>
Cc: Aleksa Sarai <>,
 Mickaël Salaün <>,
 Florian Weimer <>,
 Mickaël Salaün <>,, Alexei Starovoitov <>,
 Al Viro <>, Andy Lutomirski <>,
 Christian Heimes <>,
 Daniel Borkmann <>,
 Eric Chiang <>, James Morris <>,
 Jan Kara <>, Jann Horn <>,
 Jonathan Corbet <>, Kees Cook <>,
 Matthew Garrett <>, Matthew Wilcox <>,
 Michael Kerrisk <>, Mimi Zohar <>,
 Philippe Trébuchet <>,
 Scott Shell <>,
 Sean Christopherson <>,
 Shuah Khan <>, Song Liu <>,
 Steve Dower <>, Steve Grubb <>,
 Thibaut Sautereau <>,
 Vincent Strubel <>,
 Yves-Alexis Perez <>,,,,
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

> On Sep 6, 2019, at 12:43 PM, Jeff Layton <> wrote:
>> On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
>>> On 2019-09-06, 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.
>> 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.

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