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From: Matthew Wilcox <willy@infradead.org>
To: Mimi Zohar <zohar@linux.ibm.com>
Cc: "Mickaël Salaün" <mic@digikod.net>,
	"Jann Horn" <jannh@google.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"kernel list" <linux-kernel@vger.kernel.org>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Eric Biggers" <ebiggers@kernel.org>,
	"Eric Chiang" <ericchiang@google.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"Matthew Garrett" <mjg59@google.com>,
	"Michael Kerrisk" <mtk.manpages@gmail.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>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Tetsuo Handa" <penguin-kernel@i-love.sakura.ne.jp>,
	"Thibaut Sautereau" <thibaut.sautereau@clip-os.org>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	"Kernel Hardening" <kernel-hardening@lists.openwall.com>,
	"Linux API" <linux-api@vger.kernel.org>,
	linux-integrity@vger.kernel.org,
	linux-security-module <linux-security-module@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v7 0/7] Add support for O_MAYEXEC
Date: Tue, 11 Aug 2020 15:02:03 +0100
Message-ID: <20200811140203.GQ17456@casper.infradead.org> (raw)
In-Reply-To: <5db0ef9cb5e7e1569a5a1f7a0594937023f7290b.camel@linux.ibm.com>

On Tue, Aug 11, 2020 at 09:56:50AM -0400, Mimi Zohar wrote:
> On Tue, 2020-08-11 at 10:48 +0200, Mickaël Salaün wrote:
> > On 11/08/2020 01:03, Jann Horn wrote:
> > > On Tue, Aug 11, 2020 at 12:43 AM Mickaël Salaün <mic@digikod.net> wrote:
> > > > On 10/08/2020 22:21, Al Viro wrote:
> > > > > On Mon, Aug 10, 2020 at 10:11:53PM +0200, Mickaël Salaün wrote:
> > > > > > It seems that there is no more complains nor questions. Do you want me
> > > > > > to send another series to fix the order of the S-o-b in patch 7?
> > > > > 
> > > > > There is a major question regarding the API design and the choice of
> > > > > hooking that stuff on open().  And I have not heard anything resembling
> > > > > a coherent answer.
> > > > 
> > > > Hooking on open is a simple design that enables processes to check files
> > > > they intend to open, before they open them. From an API point of view,
> > > > this series extends openat2(2) with one simple flag: O_MAYEXEC. The
> > > > enforcement is then subject to the system policy (e.g. mount points,
> > > > file access rights, IMA, etc.).
> > > > 
> > > > Checking on open enables to not open a file if it does not meet some
> > > > requirements, the same way as if the path doesn't exist or (for whatever
> > > > reasons, including execution permission) if access is denied.
> > > 
> > > You can do exactly the same thing if you do the check in a separate
> > > syscall though.
> > > 
> > > And it provides a greater degree of flexibility; for example, you can
> > > use it in combination with fopen() without having to modify the
> > > internals of fopen() or having to use fdopen().
> > > 
> > > > It is a
> > > > good practice to check as soon as possible such properties, and it may
> > > > enables to avoid (user space) time-of-check to time-of-use (TOCTOU)
> > > > attacks (i.e. misuse of already open resources).
> > > 
> > > The assumption that security checks should happen as early as possible
> > > can actually cause security problems. For example, because seccomp was
> > > designed to do its checks as early as possible, including before
> > > ptrace, we had an issue for a long time where the ptrace API could be
> > > abused to bypass seccomp filters.
> > > 
> > > Please don't decide that a check must be ordered first _just_ because
> > > it is a security check. While that can be good for limiting attack
> > > surface, it can also create issues when the idea is applied too
> > > broadly.
> > 
> > I'd be interested with such security issue examples.
> > 
> > I hope that delaying checks will not be an issue for mechanisms such as
> > IMA or IPE:
> > https://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.com/
> > 
> > Any though Mimi, Deven, Chrome OS folks?
> 
> One of the major gaps, defining a system wide policy requiring all code
> being executed to be signed, is interpreters.  The kernel has no
> context for the interpreter's opening the file.  From an IMA
> perspective, this information needs to be conveyed to the kernel prior
> to ima_file_check(), which would allow IMA policy rules to be defined
> in terms of O_MAYEXEC.

This is kind of evading the point -- Mickaël is proposing a new flag
to open() to tell IMA to measure the file being opened before the fd
is returned to userspace, and Al is suggesting a new syscall to allow
a previously-obtained fd to be measured.

I think what you're saying is that you don't see any reason to prefer
one over the other.

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-23 17:12 Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 1/7] exec: Change uselib(2) IS_SREG() failure to EACCES Mickaël Salaün
2020-08-11 18:59   ` Eric W. Biederman
2020-08-11 19:14     ` Eric W. Biederman
2020-07-23 17:12 ` [PATCH v7 2/7] exec: Move S_ISREG() check earlier Mickaël Salaün
2020-08-11 19:27   ` Eric W. Biederman
2020-07-23 17:12 ` [PATCH v7 3/7] exec: Move path_noexec() " Mickaël Salaün
2020-08-11 19:36   ` Eric W. Biederman
2020-08-13 15:31     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 4/7] fs: Introduce O_MAYEXEC flag for openat2(2) Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-07-27  4:21   ` Al Viro
2020-07-27  5:27     ` Florian Weimer
2020-07-27 19:46       ` Mickaël Salaün
2020-08-11 19:51   ` Eric W. Biederman
2020-08-13 14:36     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-08-11 19:58   ` Eric W. Biederman
2020-08-13 14:49     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 6/7] selftest/openat2: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-07-23 17:12 ` [PATCH v7 7/7] ima: add policy support for the new file open MAY_OPENEXEC flag Mickaël Salaün
2020-07-24 19:04   ` Kees Cook
2020-07-24 11:20 ` [PATCH v7 0/7] Add support for O_MAYEXEC Thibaut Sautereau
2020-07-24 19:06 ` Kees Cook
2020-07-25 11:15   ` Christian Brauner
2020-08-10 20:11   ` Mickaël Salaün
2020-08-10 20:21     ` Al Viro
2020-08-10 22:09       ` David Laight
2020-08-10 22:28         ` Al Viro
2020-08-10 22:47           ` Mickaël Salaün
2020-08-11  8:09             ` David Laight
2020-08-11  8:50               ` Mickaël Salaün
2020-08-10 22:43       ` Mickaël Salaün
2020-08-10 23:03         ` Jann Horn
2020-08-11  8:48           ` Mickaël Salaün
2020-08-11 13:56             ` Mimi Zohar
2020-08-11 14:02               ` Matthew Wilcox [this message]
2020-08-11 14:30                 ` Mimi Zohar
2020-08-11 17:18             ` Deven Bowers
2020-08-10 23:05         ` Al Viro
2020-08-11  8:49           ` Mickaël Salaün

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