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From: "Mickaël Salaün" <mic@digikod.net>
To: Florian Weimer <fweimer@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-kernel@vger.kernel.org, "Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Eric Biggers" <ebiggers@kernel.org>,
	"Eric Chiang" <ericchiang@google.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Kees Cook" <keescook@chromium.org>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Tetsuo Handa" <penguin-kernel@i-love.sakura.ne.jp>,
	"Thibaut Sautereau" <thibaut.sautereau@clip-os.org>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>
Subject: Re: [PATCH v7 4/7] fs: Introduce O_MAYEXEC flag for openat2(2)
Date: Mon, 27 Jul 2020 21:46:55 +0200
Message-ID: <eaf5bc42-e086-740b-a90c-93e67c535eee@digikod.net> (raw)
In-Reply-To: <87y2n55xzv.fsf@oldenburg2.str.redhat.com>


On 27/07/2020 07:27, Florian Weimer wrote:
> * Al Viro:
> 
>> On Thu, Jul 23, 2020 at 07:12:24PM +0200, Mickaël Salaün wrote:
>>> When the O_MAYEXEC flag is passed, openat2(2) may be subject to
>>> additional restrictions depending on a security policy managed by the
>>> kernel through a sysctl or implemented by an LSM thanks to the
>>> inode_permission hook.  This new flag is ignored by open(2) and
>>> openat(2) because of their unspecified flags handling.  When used with
>>> openat2(2), the default behavior is only to forbid to open a directory.
>>
>> Correct me if I'm wrong, but it looks like you are introducing a magical
>> flag that would mean "let the Linux S&M take an extra special whip
>> for this open()".

There is nothing magic, it doesn't only work with the LSM framework, and
there is nothing painful nor humiliating here (except maybe this language).

>>
>> Why is it done during open?  If the caller is passing it deliberately,
>> why not have an explicit request to apply given torture device to an
>> already opened file?  Why not sys_masochism(int fd, char *hurt_flavour),
>> for that matter?
> 
> While I do not think this is appropriate language for a workplace, Al
> has a point: If the auditing event can be generated on an already-open
> descriptor, it would also cover scenarios like this one:
> 
>   perl < /path/to/script
> 
> Where the process that opens the file does not (and cannot) know that it
> will be used for execution purposes.

The check is done during open because the goal of this patch series is
to address the problem of script execution when opening a script in well
controlled systems (e.g. to enforce a "write xor execute" policy, to do
an atomic integrity check [1], to check specific execute/read
permissions, etc.). As discussed multiple times, controlling other means
to interpret commands (stdin, environment variables, etc.) is out of
scope and should be handled by interpreters (in userspace). Someone
could still extend fcntl(2) to enable to check file descriptors, but it
is an independent change not required for now.
Specific audit features are also out of scope for now [2].

[1] https://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.com/
[2] https://lore.kernel.org/lkml/202007160822.CCDB5478@keescook/

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-23 17:12 [PATCH v7 0/7] Add support for O_MAYEXEC Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 1/7] exec: Change uselib(2) IS_SREG() failure to EACCES Mickaël Salaün
2020-08-11 18:59   ` Eric W. Biederman
2020-08-11 19:14     ` Eric W. Biederman
2020-07-23 17:12 ` [PATCH v7 2/7] exec: Move S_ISREG() check earlier Mickaël Salaün
2020-08-11 19:27   ` Eric W. Biederman
2020-07-23 17:12 ` [PATCH v7 3/7] exec: Move path_noexec() " Mickaël Salaün
2020-08-11 19:36   ` Eric W. Biederman
2020-08-13 15:31     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 4/7] fs: Introduce O_MAYEXEC flag for openat2(2) Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-07-27  4:21   ` Al Viro
2020-07-27  5:27     ` Florian Weimer
2020-07-27 19:46       ` Mickaël Salaün [this message]
2020-08-11 19:51   ` Eric W. Biederman
2020-08-13 14:36     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-08-11 19:58   ` Eric W. Biederman
2020-08-13 14:49     ` Mickaël Salaün
2020-07-23 17:12 ` [PATCH v7 6/7] selftest/openat2: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2020-07-24 19:03   ` Kees Cook
2020-07-23 17:12 ` [PATCH v7 7/7] ima: add policy support for the new file open MAY_OPENEXEC flag Mickaël Salaün
2020-07-24 19:04   ` Kees Cook
2020-07-24 11:20 ` [PATCH v7 0/7] Add support for O_MAYEXEC Thibaut Sautereau
2020-07-24 19:06 ` Kees Cook
2020-07-25 11:15   ` Christian Brauner
2020-08-10 20:11   ` Mickaël Salaün
2020-08-10 20:21     ` Al Viro
2020-08-10 22:09       ` David Laight
2020-08-10 22:28         ` Al Viro
2020-08-10 22:47           ` Mickaël Salaün
2020-08-11  8:09             ` David Laight
2020-08-11  8:50               ` Mickaël Salaün
2020-08-10 22:43       ` Mickaël Salaün
2020-08-10 23:03         ` Jann Horn
2020-08-11  8:48           ` Mickaël Salaün
2020-08-11 13:56             ` Mimi Zohar
2020-08-11 14:02               ` Matthew Wilcox
2020-08-11 14:30                 ` Mimi Zohar
2020-08-11 17:18             ` Deven Bowers
2020-08-10 23:05         ` Al Viro
2020-08-11  8:49           ` Mickaël Salaün

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