linux-kernel - Re: [RFC PATCH v9 0/3] Add introspect_access(2) (was O

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Date:   Thu, 10 Sep 2020 14:08:56 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Mickaël Salaün <mic@...ikod.net>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Eric Chiang <ericchiang@...gle.com>,
        Florian Weimer <fweimer@...hat.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>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH v9 0/3] Add introspect_access(2) (was O_MAYEXEC)

On Thu, 2020-09-10 at 19:21 +0200, Mickaël Salaün wrote:
> On 10/09/2020 19:04, Matthew Wilcox wrote:
> > On Thu, Sep 10, 2020 at 06:46:09PM +0200, Mickaël Salaün wrote:
> >> This ninth patch series rework the previous AT_INTERPRETED and O_MAYEXEC
> >> series with a new syscall: introspect_access(2) .  Access check are now
> >> only possible on a file descriptor, which enable to avoid possible race
> >> conditions in user space.
> > 
> > But introspection is about examining _yourself_.  This isn't about
> > doing that.  It's about doing ... something ... to a script that you're
> > going to execute.  If the script were going to call the syscall, then
> > it might be introspection.  Or if the interpreter were measuring itself,
> > that would be introspection.  But neither of those would be useful things
> > to do, because an attacker could simply avoid doing them.
> 

Michael, is the confusion here that IMA isn't measuring anything, but
verifying the integrity of the file?   The usecase, from an IMA
perspective, is enforcing a system wide policy requiring everything
executed to be signed.  In this particular use case, the interpreter is
asking the kernel if the script is signed with a permitted key.  The
signature may be an IMA signature or an EVM portable and immutable
signature, based on policy.

> Picking a good name other than "access" (or faccessat2) is not easy. The
> idea with introspect_access() is for the calling task to ask the kernel
> if this task should allows to do give access to a kernel resource which
> is already available to this task. In this sense, we think that
> introspection makes sense because it is the choice of the task to allow
> or deny an access.
> 
> > 
> > So, bad name.  What might be better?  sys_security_check()?
> > sys_measure()?  sys_verify_fd()?  I don't know.
> > 
> 
> "security_check" looks quite broad, "measure" doesn't make sense here,
> "verify_fd" doesn't reflect that it is an access check. Yes, not easy,
> but if this is the only concern we are on the good track. :)

Maybe replacing the term "measure" with "integrity", but rather than
"integrity_check", something along the lines of fgetintegrity,
freadintegrity, fcheckintegrity.

Mimi

> 
> 
> Other ideas:
> - interpret_access (mainly, but not only, for interpreters)
> - indirect_access
> - may_access
> - faccessat3