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Message-ID: <CALmYWFuOXAiT05Pi2rZ1nUAKDGe9JyTH7fro2EYS1fh3zeGV5Q@mail.gmail.com>
Date: Tue, 9 Jul 2024 14:57:43 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Steve Dower <steve.dower@...hon.org>, Al Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Kees Cook <keescook@...omium.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 Tue, Jul 9, 2024 at 1:42 PM Mickaël Salaün <mic@...ikod.net> wrote:
>
> On Mon, Jul 08, 2024 at 03:07:24PM -0700, Jeff Xu wrote:
> > On Mon, Jul 8, 2024 at 2:25 PM Steve Dower <steve.dower@...hon.org> wrote:
> > >
> > > On 08/07/2024 22:15, Jeff Xu wrote:
> > > > IIUC:
> > > > CHECK=0, RESTRICT=0: do nothing, current behavior
> > > > CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results.
> > > > CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception.
> > > > CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except
> > > > those in the "checked-and-allowed" list.
> > >
> > > I had much the same question for Mickaël while working on this.
> > >
> > > Essentially, "CHECK=0, RESTRICT=1" means to restrict without checking.
> > > In the context of a script or macro interpreter, this just means it will
> > > never interpret any scripts. Non-binary code execution is fully disabled
> > > in any part of the process that respects these bits.
> > >
> > I see, so Mickaël does mean this will block all scripts.
>
> That is the initial idea.
>
> > I guess, in the context of dynamic linker, this means: no more .so
> > loading, even "dlopen" is called by an app ?  But this will make the
> > execve()  fail.
>
> Hmm, I'm not sure this "CHECK=0, RESTRICT=1" configuration would make
> sense for a dynamic linker except maybe if we want to only allow static
> binaries?
>
> The CHECK and RESTRICT securebits are designed to make it possible a
> "permissive mode" and an enforcement mode with the related locked
> securebits.  This is why this "CHECK=0, RESTRICT=1" combination looks a
> bit weird.  We can replace these securebits with others but I didn't
> find a better (and simple) option.  I don't think this is an issue
> because with any security policy we can create unusable combinations.
> The three other combinations makes a lot of sense though.
>
If we need only handle 3  combinations,  I would think something like
below is easier to understand, and don't have wield state like
CHECK=0, RESTRICT=1

XX_RESTRICT: when true: Perform the AT_CHECK, and deny the executable
after AT_CHECK fails.
XX_RESTRICT_PERMISSIVE:  take effect when XX_RESTRICT is true. True
means Ignoring the AT_CHECK result.

Or

XX_CHECK: when true: Perform the AT_CHECK.
XX_CHECK_ENFORCE takes effect only when XX_CHECK is true.   True means
restrict the executable when AT_CHECK failed; false means ignore the
AT_CHECK failure.

Of course, we can replace XX_CHECK_ENFORCE with XX_RESTRICT.
Personally I think having _CHECK_ in the name implies the XX_CHECK
needs to be true as a prerequisite for this flag , but that is my
opinion only. As long as the semantics are clear as part of the
comments of definition in code,  it is fine.

Thanks
-Jeff


> >
> > > "CHECK=1, RESTRICT=1" means to restrict unless AT_CHECK passes. This
> > > case is the allow list (or whatever mechanism is being used to determine
> > > the result of an AT_CHECK check). The actual mechanism isn't the
> > > business of the script interpreter at all, it just has to refuse to
> > > execute anything that doesn't pass the check. So a generic interpreter
> > > can implement a generic mechanism and leave the specifics to whoever
> > > configures the machine.
> > >
> > In the context of dynamic linker. this means:
> > if .so passed the AT_CHECK, ldopen() can still load it.
> > If .so fails the AT_CHECK, ldopen() will fail too.
>
> Correct
>
> >
> > Thanks
> > -Jeff
> >
> > > The other two case are more obvious. "CHECK=0, RESTRICT=0" is the
> > > zero-overhead case, while "CHECK=1, RESTRICT=0" might log, warn, or
> > > otherwise audit the result of the check, but it won't restrict execution.
> > >
> > > Cheers,
> > > Steve

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