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Date: Fri, 1 May 2020 13:47:24 +0200
From: Christian Heimes <christian@...hon.org>
To: Jann Horn <jannh@...gle.com>, Florian Weimer <fw@...eb.enyo.de>
Cc: Mickaël Salaün <mic@...ikod.net>,
kernel list <linux-kernel@...r.kernel.org>, Aleksa Sarai
<cyphar@...har.com>, Alexei Starovoitov <ast@...nel.org>,
Al Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Deven Bowers <deven.desai@...ux.microsoft.com>,
Eric Chiang <ericchiang@...gle.com>, James Morris <jmorris@...ei.org>,
Jan Kara <jack@...e.cz>, Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>,
Matthew Wilcox <willy@...radead.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Mickaël Salaün <mickael.salaun@....gouv.fr>,
Mimi Zohar <zohar@...ux.ibm.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>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux API <linux-api@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 0/5] Add support for RESOLVE_MAYEXEC
On 29/04/2020 00.01, Jann Horn wrote:
> On Tue, Apr 28, 2020 at 11:21 PM Florian Weimer <fw@...eb.enyo.de> wrote:
>> * Jann Horn:
>>
>>> Just as a comment: You'd probably also have to use RESOLVE_MAYEXEC in
>>> the dynamic linker.
>>
>> Absolutely. In typical configurations, the kernel does not enforce
>> that executable mappings must be backed by files which are executable.
>> It's most obvious with using an explicit loader invocation to run
>> executables on noexec mounts. RESOLVE_MAYEXEC is much more useful
>> than trying to reimplement the kernel permission checks (or what some
>> believe they should be) in userspace.
>
> Oh, good point.
>
> That actually seems like something Mickaël could add to his series? If
> someone turns on that knob for "When an interpreter wants to execute
> something, enforce that we have execute access to it", they probably
> also don't want it to be possible to just map files as executable? So
> perhaps when that flag is on, the kernel should either refuse to map
> anything as executable if it wasn't opened with RESOLVE_MAYEXEC or
> (less strict) if RESOLVE_MAYEXEC wasn't used, print a warning, then
> check whether the file is executable and bail out if not?
>
> A configuration where interpreters verify that scripts are executable,
> but other things can just mmap executable pages, seems kinda
> inconsistent...
+1
I worked with Steve Downer on Python PEP 578 [1] that added audit hooks
and PyFile_OpenCode() to CPython. A PyFile_OpenCode() implementation
with RESOLVE_MAYEXEC will hep to secure loading of Python code. But
Python also includes a wrapper of libffi. ctypes or cffi can load native
code from either shared libraries with dlopen() or execute native code
from mmap() regions. For example SnakeEater [2] is a clever attack that
abused memfd_create syscall and proc filesystem to execute code.
A consistent security policy must also ensure that mmap() PROT_EXEC
enforces the same restrictions as RESOLVE_MAYEXEC. The restriction
doesn't have be part of this patch, though.
Christian
[1] https://www.python.org/dev/peps/pep-0578/
[2] https://github.com/nullbites/SnakeEater/blob/master/SnakeEater2.py
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