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From: "Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>
To: Christian Brauner <christian.brauner@ubuntu.com>,
	Aleksa Sarai <cyphar@cyphar.com>
Cc: "Florian Weimer" <fweimer@redhat.com>,
	"Mickaël Salaün" <mic@digikod.net>,
	linux-kernel@vger.kernel.org,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Eric Chiang" <ericchiang@google.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Kees Cook" <keescook@chromium.org>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Song Liu" <songliubraving@fb.com>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	"Yves-Alexis Perez" <yves-alexis.perez@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()
Date: Fri, 6 Sep 2019 19:24:25 +0200
Message-ID: <062c25e2-1996-c32c-46a1-aa831d69930f@ssi.gouv.fr> (raw)
In-Reply-To: <20190906172050.v44f43psd6qc6awi@wittgenstein>


On 06/09/2019 19:20, Christian Brauner wrote:
> On Sat, Sep 07, 2019 at 03:07:39AM +1000, Aleksa Sarai wrote:
>> On 2019-09-06, Mickaël Salaün <mickael.salaun@ssi.gouv.fr> 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.
>>
>> Depends on your definition of "safe" -- a security feature that you will
>> silently not enable on older kernels doesn't sound super safe to me.
>> Unfortunately this is a limitation of open(2) that we cannot change --
>> which is why the openat2(2) proposal I've been posting gives -EINVAL for
>> unknown O_* flags.
>>
>> There is a way to probe for support (though unpleasant), by creating a
>> test O_MAYEXEC fd and then checking if the flag is present in
>> /proc/self/fdinfo/$n.
>
> Which Florian said they can't do for various reasons.
>
> It is a major painpoint if there's no easy way for userspace to probe
> for support. Especially if it's security related which usually means
> that you want to know whether this feature works or not.

I used "safe" deliberately (not "secure" which didn't make sense in this
sentence). According to the threat model, if the kernel doesn't support
the feature, it should be ignored by userland. In this case, it fit well
with the current behavior of open(2). I agree that the openat2(2)
behavior handling flags is the good way to do it (whitelisting), but the
O_MAYEXEC flag should not change the userland behavior on its own,
because it depend on a global policy. Even being able to probe for
O_MAYEXEC support does not make sense because it would not be enough to
know the system policy (either this flag is enforced or not…).

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  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-06 15:24 [PATCH v2 0/5] Add support for O_MAYEXEC Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Mickaël Salaün
2019-09-06 15:56   ` Florian Weimer
2019-09-06 16:06     ` Mickaël Salaün
2019-09-06 16:48       ` Jeff Layton
2019-09-06 17:13         ` Aleksa Sarai
2019-09-06 19:43           ` Jeff Layton
2019-09-06 20:06             ` Andy Lutomirski
2019-09-06 20:51               ` Jeff Layton
2019-09-06 21:27                 ` Andy Lutomirski
2019-09-06 22:12                 ` Aleksa Sarai
2019-09-09  9:33               ` Mickaël Salaün
2019-09-06 22:05             ` Aleksa Sarai
2019-09-06 22:18               ` Aleksa Sarai
2019-09-06 17:14         ` Mickaël Salaün
2019-09-06 18:38           ` Jeff Layton
2019-09-06 18:41             ` Andy Lutomirski
2019-09-09  9:18               ` Mickaël Salaün
2019-09-09 15:49                 ` Andy Lutomirski
2019-09-06 18:44             ` Florian Weimer
2019-09-06 19:03             ` James Morris
2019-09-09  9:25               ` Mickaël Salaün
2019-09-09 10:12                 ` James Morris
2019-09-09 10:54                   ` Mickaël Salaün
2019-09-09 12:28                     ` Aleksa Sarai
2019-09-09 12:33                       ` Mickaël Salaün
2019-09-09 11:54                 ` Aleksa Sarai
2019-09-09 12:28                   ` Mickaël Salaün
2019-09-06 17:07       ` Aleksa Sarai
2019-09-06 17:20         ` Christian Brauner
2019-09-06 17:24           ` Mickaël Salaün [this message]
2019-09-06 17:40           ` Tycho Andersen
2019-09-06 18:27             ` Florian Weimer
2019-09-06 18:46               ` Tycho Andersen
2019-09-06 15:24 ` [PATCH v2 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 3/5] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 4/5] selftest/exec: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 5/5] doc: Add documentation for the fs.open_mayexec_enforce sysctl Mickaël Salaün
2019-09-06 18:50 ` [PATCH v2 0/5] Add support for O_MAYEXEC Steve Grubb
2019-09-06 18:57   ` Florian Weimer
2019-09-06 19:07     ` Steve Grubb
2019-09-06 19:26       ` Andy Lutomirski
2019-09-06 22:44         ` Aleksa Sarai
2019-09-09  9:09           ` Mickaël Salaün
2019-09-09  0:16 ` James Morris

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