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Subject: Re: [PATCH v3 0/5] Add support for RESOLVE_MAYEXEC
To:     Jann Horn <jannh@google.com>, Florian Weimer <fw@deneb.enyo.de>
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        Aleksa Sarai <cyphar@cyphar.com>,
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        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mickael.salaun@ssi.gouv.fr>,
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        Vincent Strubel <vincent.strubel@ssi.gouv.fr>,
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        Linux API <linux-api@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
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Date:   Wed, 29 Apr 2020 10:50:19 +0200
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On 29/04/2020 00:01, Jann Horn wrote:
> On Tue, Apr 28, 2020 at 11:21 PM Florian Weimer <fw@deneb.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.

Indeed it makes sense to use RESOLVE_MAYEXEC for the dynamic linker too.
Only the noexec mount option is taken into account for mmap(2) with
PROT_EXEC, and if you can trick the dynamic linker with JOP as Jann
explained, it may enable to execute new code. However, a kernel which
forbids remapping memory with PROT_EXEC still enables to implement a W^X
policy. Any JOP/ROP still enables unexpected code execution though.

> 
> 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...

As it is written in the documentation patch, this RESOLVE_MAYEXEC
feature is an important missing piece, but to implement a consistent
security policy we need to enable other restrictions starting with a
noexec mount point policy. The purpose of this patch series is not to
bring a full-feature LSM with process states handling, but it brings
what is needed for LSMs such as SELinux, IMA or IPE to extend their
capabilities to reach what you would expect.