linux-hardening - Re: [RESEND RFC] kernel/ksysfs.c: restrict /sys/kernel/notes to root access

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Date: Mon, 19 Feb 2024 14:07:57 +0100
From: Jürgen Groß <jgross@...e.com>
To: Kees Cook <keescook@...omium.org>, Greg KH <gregkh@...uxfoundation.org>
Cc: Guixiong Wei <guixiongwei@...il.com>, linux-hardening@...r.kernel.org,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Guixiong Wei <weiguixiong@...edance.com>, linux-kernel@...r.kernel.org
Subject: Re: [RESEND RFC] kernel/ksysfs.c: restrict /sys/kernel/notes to root
 access

On 18.02.24 10:04, Kees Cook wrote:
> On Sun, Feb 18, 2024 at 08:47:03AM +0100, Greg KH wrote:
>> On Sun, Feb 18, 2024 at 03:35:01PM +0800, Guixiong Wei wrote:
>>> From: Guixiong Wei <weiguixiong@...edance.com>
>>>
>>> Restrict non-privileged user access to /sys/kernel/notes to
>>> avoid security attack.
>>>
>>> The non-privileged users have read access to notes. The notes
>>> expose the load address of startup_xen. This address could be
>>> used to bypass KASLR.
>>
>> How can it be used to bypass it?
>>
>> KASLR is, for local users, pretty much not an issue, as that's not what
>> it protects from, only remote ones.
>>
>>> For example, the startup_xen is built at 0xffffffff82465180 and
>>> commit_creds is built at 0xffffffff810ad570 which could read from
>>> the /boot/System.map. And the loaded address of startup_xen is
>>> 0xffffffffbc265180 which read from /sys/kernel/notes. So the loaded
>>> address of commit_creds is 0xffffffffbc265180 - (0xffffffff82465180
>>>   - 0xffffffff810ad570) = 0xffffffffbaead570.
>>
>> I've cc: the hardening list on this, I'm sure the developers there have
>> opinions about this.
> 
> Oh eww, why is Xen spewing addresses into the notes section? (This must
> be how it finds its entry point? But that would be before relocations
> happen...)

Right. Xen is looking into the ELF-file to find the entry point of the
kernel (PV and PVH guest types only).

> 
> But yes, I can confirm that relocations are done against the .notes
> section at boot, so the addresses exposed in .notes is an immediate
> KASLR offset exposure.

Relocations applied to the kernel when it has been started don't need
to cover the notes section as far as Xen is concerned.


Juergen