[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CALmYWFt7X0v8k1N9=aX6BuT2gCiC9SeWwPEBckvBk8GQtb0rqQ@mail.gmail.com>
Date: Tue, 16 Jul 2024 08:02:37 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Kees Cook <kees@...nel.org>, Steve Dower <steve.dower@...hon.org>,
Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>, Paul Moore <paul@...l-moore.com>,
"Theodore Ts'o" <tytso@....edu>, Alejandro Colomar <alx@...nel.org>, Aleksa Sarai <cyphar@...har.com>,
Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>,
Arnd Bergmann <arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>,
Christian Heimes <christian@...hon.org>, Dmitry Vyukov <dvyukov@...gle.com>,
Eric Biggers <ebiggers@...nel.org>, Eric Chiang <ericchiang@...gle.com>,
Fan Wu <wufan@...ux.microsoft.com>, Florian Weimer <fweimer@...hat.com>,
Geert Uytterhoeven <geert@...ux-m68k.org>, James Morris <jamorris@...ux.microsoft.com>,
Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
Jordan R Abrahams <ajordanr@...gle.com>, Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
Luca Boccassi <bluca@...ian.org>, Luis Chamberlain <mcgrof@...nel.org>,
"Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>, Matt Bobrowski <mattbobrowski@...gle.com>,
Matthew Garrett <mjg59@...f.ucam.org>, Matthew Wilcox <willy@...radead.org>,
Miklos Szeredi <mszeredi@...hat.com>, Mimi Zohar <zohar@...ux.ibm.com>,
Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>,
Shuah Khan <shuah@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>, Steve Grubb <sgrubb@...hat.com>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>, Xiaoming Ni <nixiaoming@...wei.com>,
Yin Fengwei <fengwei.yin@...el.com>, kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and
SHOULD_EXEC_RESTRICT securebits
On Thu, Jul 11, 2024 at 1:57 AM Mickaël Salaün <mic@...ikod.net> wrote:
>
> On Wed, Jul 10, 2024 at 09:26:14AM -0700, Kees Cook wrote:
> > On Wed, Jul 10, 2024 at 11:58:25AM +0200, Mickaël Salaün wrote:
> > > Here is another proposal:
> > >
> > > We can change a bit the semantic by making it the norm to always check
> > > file executability with AT_CHECK, and using the securebits to restrict
> > > file interpretation and/or command injection (e.g. user supplied shell
> > > commands). Non-executable checked files can be reported/logged at the
> > > kernel level, with audit, configured by sysadmins.
> > >
> > > New securebits (feel free to propose better names):
> > >
> > > - SECBIT_EXEC_RESTRICT_FILE: requires AT_CHECK to pass.
> >
> > Would you want the enforcement of this bit done by userspace or the
> > kernel?
> >
> > IIUC, userspace would always perform AT_CHECK regardless of
> > SECBIT_EXEC_RESTRICT_FILE, and then which would happen?
> >
> > 1) userspace would ignore errors from AT_CHECK when
> > SECBIT_EXEC_RESTRICT_FILE is unset
>
> Yes, that's the idea.
>
> >
> > or
> >
> > 2) kernel would allow all AT_CHECK when SECBIT_EXEC_RESTRICT_FILE is
> > unset
> >
> > I suspect 1 is best and what you intend, given that
> > SECBIT_EXEC_DENY_INTERACTIVE can only be enforced by userspace.
>
> Indeed. We don't want AT_CHECK's behavior to change according to
> securebits.
>
One bit is good.
> >
> > > - SECBIT_EXEC_DENY_INTERACTIVE: deny any command injection via
> > > command line arguments, environment variables, or configuration files.
> > > This should be ignored by dynamic linkers. We could also have an
> > > allow-list of shells for which this bit is not set, managed by an
> > > LSM's policy, if the native securebits scoping approach is not enough.
> > >
> > > Different modes for script interpreters:
> > >
> > > 1. RESTRICT_FILE=0 DENY_INTERACTIVE=0 (default)
> > > Always interpret scripts, and allow arbitrary user commands.
> > > => No threat, everyone and everything is trusted, but we can get
> > > ahead of potential issues with logs to prepare for a migration to a
> > > restrictive mode.
> > >
> > > 2. RESTRICT_FILE=1 DENY_INTERACTIVE=0
> > > Deny script interpretation if they are not executable, and allow
> > > arbitrary user commands.
> > > => Threat: (potential) malicious scripts run by trusted (and not
> > > fooled) users. That could protect against unintended script
> > > executions (e.g. sh /tmp/*.sh).
> > > ==> Makes sense for (semi-restricted) user sessions.
> > >
> > > 3. RESTRICT_FILE=1 DENY_INTERACTIVE=1
> > > Deny script interpretation if they are not executable, and also deny
> > > any arbitrary user commands.
> > > => Threat: malicious scripts run by untrusted users.
> > > ==> Makes sense for system services executing scripts.
> > >
> > > 4. RESTRICT_FILE=0 DENY_INTERACTIVE=1
> > > Always interpret scripts, but deny arbitrary user commands.
> > > => Goal: monitor/measure/assess script content (e.g. with IMA/EVM) in
> > > a system where the access rights are not (yet) ready. Arbitrary
> > > user commands would be much more difficult to monitor.
> > > ==> First step of restricting system services that should not
> > > directly pass arbitrary commands to shells.
> >
> > I like these bits!
>
> Good! Jeff, Steve, Florian, Matt, others, what do you think?
For below two cases: will they be restricted by one (or some) mode above ?
1> cat /tmp/a.sh | sh
2> sh -c "$(cat /tmp/a.sh)"
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
