linux-kernel - Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys

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Message-Id: <D2A57C7B-B0FD-424E-9F81-B858FFF21FF0@amacapital.net>
Date:   Fri, 6 Sep 2019 13:06:00 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        Florian Weimer <fweimer@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Chiang <ericchiang@...gle.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Song Liu <songliubraving@...com>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
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 <jlayton@...nel.org> wrote:
> 
>> On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
>>> On 2019-09-06, Jeff Layton <jlayton@...nel.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.