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

From: Andy Lutomirski <luto@amacapital.net>
To: Jeff Layton <jlayton@kernel.org>
Cc: "Aleksa Sarai" <cyphar@cyphar.com>,
	"Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"Florian Weimer" <fweimer@redhat.com>,
	"Mickaël Salaün" <mic@digikod.net>,
	LKML <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" <kernel-hardening@lists.openwall.com>,
	"Linux API" <linux-api@vger.kernel.org>,
	"LSM List" <linux-security-module@vger.kernel.org>,
	"Linux FS Devel" <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 14:27:19 -0700	[thread overview]
Message-ID: <CALCETrVR5d2XTpAN8QLRv3cYDfpAdZRNNcD-TtE5H+v7-i7QhQ@mail.gmail.com> (raw)
In-Reply-To: <8dc59d585a133e96f9adaf0a148334e7f19058b9.camel@kernel.org>

> On Sep 6, 2019, at 1:51 PM, Jeff Layton <jlayton@kernel.org> wrote:
>
> On Fri, 2019-09-06 at 13:06 -0700, Andy Lutomirski wrote:
>
>> I’m not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO.
>
> Good point. Upgradability is definitely orthogonal, though the idea
> there is to alter the default behavior. If the default is NOEXEC then
> UPGRADE_EXEC would make sense.
>
> In any case, I was mostly thinking about the middle two in your list
> above. After more careful reading of the patches, I now get get that
> Mickaël is more interested in the first, and that's really a different
> sort of use-case.
>
> Most opens never result in the fd being fed to fexecve or mmapped with
> PROT_EXEC, so having userland explicitly opt-in to allowing that during
> the open sounds like a reasonable thing to do.
>
> But I get that preventing execution via script interpreters of files
> that are not executable might be something nice to have.
>
> Perhaps we need two flags for openat2?
>
> OA2_MAYEXEC : test that permissions allow execution and that the file
> doesn't reside on a noexec mount before allowing the open
>
> OA2_EXECABLE : only allow fexecve or mmapping with PROT_EXEC if the fd
> was opened with this
>
>
>

We could go one step farther and have three masks: check_perms,
fd_perms, and upgrade_perms.  check_perms says “fail if I don’t have
these perms”.  fd_perms is the permissions on the returned fd, and
upgrade_perms is the upgrade mask.  (fd_perms  & ~check_perms) != 0 is
an error.  This makes it possible to say "I want to make sure the file
is writable, but I don't actually want to write to it", which could
plausibly be useful.

I would argue that these things should have new, sane bits, e.g.
FILE_READ, FILE_WRITE, and FILE_EXECUTE (or maybe FILE_MAP_EXEC and
FILE_EXECVE).  And maybe there should be at least 16 bits for each
mask reserved.  Windows has a lot more mode bits than Linux, and it's
not entirely nuts.  We do *not* need any direct equivalent of O_RDWR
for openat2().

--Andy