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Date: Fri, 6 Sep 2019 14:27:19 -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>, 
	LKML <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 <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, 
	LSM List <linux-security-module@...r.kernel.org>, 
	Linux FS Devel <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 1:51 PM, Jeff Layton <jlayton@...nel.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

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