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Date: Tue, 08 Sep 2020 11:38:28 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: Mickaël Salaün <mic@...ikod.net>,
Stephen Smalley
<stephen.smalley.work@...il.com>,
Casey Schaufler <casey@...aufler-ca.com>
Cc: linux-kernel <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>,
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>,
Matthew Wilcox <willy@...radead.org>,
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,
LSM List
<linux-security-module@...r.kernel.org>,
Linux FS Devel
<linux-fsdevel@...r.kernel.org>,
Thibaut Sautereau
<thibaut.sautereau@....gouv.fr>,
Mickaël Salaün
<mic@...ux.microsoft.com>,
John Johansen <john.johansen@...onical.com>
Subject: Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for
faccessat2(2)
[Cc'ing Casey]
On Tue, 2020-09-08 at 16:14 +0200, Mickaël Salaün wrote:
> On 08/09/2020 15:42, Stephen Smalley wrote:
> > On Tue, Sep 8, 2020 at 9:29 AM Mimi Zohar <zohar@...ux.ibm.com> wrote:
> >>
> >> On Tue, 2020-09-08 at 08:52 -0400, Stephen Smalley wrote:
> >>> On Tue, Sep 8, 2020 at 8:50 AM Stephen Smalley
> >>> <stephen.smalley.work@...il.com> wrote:
> >>>>
> >>>> On Tue, Sep 8, 2020 at 8:43 AM Mickaël Salaün <mic@...ikod.net> wrote:
> >>>>>
> >>>>>
> >>>>> On 08/09/2020 14:28, Mimi Zohar wrote:
> >>>>>> Hi Mickael,
> >>>>>>
> >>>>>> On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
> >>>>>>> + mode |= MAY_INTERPRETED_EXEC;
> >>>>>>> + /*
> >>>>>>> + * For compatibility reasons, if the system-wide policy
> >>>>>>> + * doesn't enforce file permission checks, then
> >>>>>>> + * replaces the execute permission request with a read
> >>>>>>> + * permission request.
> >>>>>>> + */
> >>>>>>> + mode &= ~MAY_EXEC;
> >>>>>>> + /* To be executed *by* user space, files must be readable. */
> >>>>>>> + mode |= MAY_READ;
> >>>>>>
> >>>>>> After this change, I'm wondering if it makes sense to add a call to
> >>>>>> security_file_permission(). IMA doesn't currently define it, but
> >>>>>> could.
> >>>>>
> >>>>> Yes, that's the idea. We could replace the following inode_permission()
> >>>>> with file_permission(). I'm not sure how this will impact other LSMs though.
> >>
> >> I wasn't suggesting replacing the existing security_inode_permission
> >> hook later, but adding a new security_file_permission hook here.
> >>
> >>>>
> >>>> They are not equivalent at least as far as SELinux is concerned.
> >>>> security_file_permission() was only to be used to revalidate
> >>>> read/write permissions previously checked at file open to support
> >>>> policy changes and file or process label changes. We'd have to modify
> >>>> the SELinux hook if we wanted to have it check execute access even if
> >>>> nothing has changed since open time.
> >>>
> >>> Also Smack doesn't appear to implement file_permission at all, so it
> >>> would skip Smack checking.
> >>
> >> My question is whether adding a new security_file_permission call here
> >> would break either SELinux or Apparmor?
> >
> > selinux_inode_permission() has special handling for MAY_ACCESS so we'd
> > need to duplicate that into selinux_file_permission() ->
> > selinux_revalidate_file_permission(). Also likely need to adjust
> > selinux_file_permission() to explicitly check whether the mask
> > includes any permissions not checked at open time. So some changes
> > would be needed here. By default, it would be a no-op unless there
> > was a policy reload or the file was relabeled between the open(2) and
> > the faccessat(2) call.
> >
>
> We could create a new hook path_permission(struct path *path, int mask)
> as a superset of inode_permission(). To be more convenient, his new hook
> could then just call inode_permission() for every LSMs not implementing
> path_permission().
The LSM maintainers need to chime in here on this suggestion. In terms
of the name, except for one hook, all the security_path_XXXX() hooks
are dependent on CONFIG_SECURITY_PATH being configured.
Mimi
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