Re: [PATCH v3 0/5] Add support for RESOLVE_MAYEXEC

From: Aleksa Sarai <cyphar@cyphar.com>
To: "Mickaël Salaün" <mic@digikod.net>
Cc: linux-kernel@vger.kernel.org,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Christian Heimes" <christian@python.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"Eric Chiang" <ericchiang@google.com>,
	"Florian Weimer" <fweimer@redhat.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>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"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>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v3 0/5] Add support for RESOLVE_MAYEXEC
Date: Thu, 30 Apr 2020 11:54:29 +1000	[thread overview]
Message-ID: <20200430015429.wuob7m5ofdewubui@yavin.dot.cyphar.com> (raw)
In-Reply-To: <20200428175129.634352-1-mic@digikod.net>

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On 2020-04-28, Mickaël Salaün <mic@digikod.net> wrote:
> The goal of this patch series is to enable to control script execution
> with interpreters help.  A new RESOLVE_MAYEXEC flag, usable through
> openat2(2), is added to enable userspace script interpreter to delegate
> to the kernel (and thus the system security policy) the permission to
> interpret/execute scripts or other files containing what can be seen as
> commands.
> 
> This third patch series mainly differ from the previous one by relying
> on the new openat2(2) system call to get rid of the undefined behavior
> of the open(2) flags.  Thus, the previous O_MAYEXEC flag is now replaced
> with the new RESOLVE_MAYEXEC flag and benefits from the openat2(2)
> strict check of this kind of flags.

My only strong upfront objection is with this being a RESOLVE_ flag.

RESOLVE_ flags have a specific meaning (they generally apply to all
components, and affect the rules of path resolution). RESOLVE_MAYEXEC
does neither of these things and so seems out of place among the other
RESOLVE_ flags.

I would argue this should be an O_ flag, but not supported for the
old-style open(2). This is what the O_SPECIFIC_FD patchset does[1] and I
think it's a reasonable way of solving such problems.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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