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From: Al Viro <viro@zeniv.linux.org.uk>
To: "Mickaël Salaün" <mic@digikod.net>
Cc: "Kees Cook" <keescook@chromium.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	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>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"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>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"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>,
	"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@lists.openwall.com, linux-api@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v7 0/7] Add support for O_MAYEXEC
Date: Tue, 11 Aug 2020 00:05:21 +0100
Message-ID: <20200810230521.GG1236603@ZenIV.linux.org.uk> (raw)
In-Reply-To: <917bb071-8b1a-3ba4-dc16-f8d7b4cc849f@digikod.net>

On Tue, Aug 11, 2020 at 12:43:52AM +0200, Mickaël Salaün wrote:

> Hooking on open is a simple design that enables processes to check files
> they intend to open, before they open them.

Which is a good thing, because...?

> 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.).

That's what "unspecified" means - as far as the kernel concerned, it's
"something completely opaque, will let these hooks to play, semantics is
entirely up to them".
 
> 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. 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).

?????  You explicitly assume a cooperating caller.  If it can't be trusted
to issue the check between open and use, or can be manipulated (ptraced,
etc.) into not doing so, how can you rely upon the flag having been passed
in the first place?  And TOCTOU window is definitely not wider that way.

If you want to have it done immediately after open(), bloody well do it
immediately after open.  If attacker has subverted your control flow to the
extent that allows them to hit descriptor table in the interval between
these two syscalls, you have already lost - they'll simply prevent that
flag from being passed.

What's the point of burying it inside openat2()?  A convenient multiplexor
to hook into?  We already have one - it's called do_syscall_...

  parent 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
2020-08-11 14:30                 ` Mimi Zohar
2020-08-11 17:18             ` Deven Bowers
2020-08-10 23:05         ` Al Viro [this message]
2020-08-11  8:49           ` Mickaël Salaün

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