Date: Tue, 8 Sep 2020 16:14:32 +0200 From: Mickaël Salaün <mic@...ikod.net> To: Stephen Smalley <stephen.smalley.work@...il.com>, Mimi Zohar <zohar@...ux.ibm.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) 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().
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