linux-kernel - Is SKAS still required for UML

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Date:	Wed, 27 Aug 2008 12:57:53 -0400 (EDT)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	jdike@...ux.intel.com
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Is SKAS still required for UML

Hi,

the UML page at http://user-mode-linux.sourceforge.net/old/skas.html 
mentions that "The UML kernel is present in the address space of each of 
its processes, and, by default, is writeable".

I tried to put this to a test, and actually failed to modify the 
UML kernel/memory image from within it. I had a simple kernel module 
with 'int val = 2;' and upon loading this, done printk("Val is at %p\n", 
&val); to get to know the address. A userspace program inside the UML 
then tried to dereference that address and read the value, but that 
ended in a segfault. /proc/xxx/maps also does not show the UML kernel 
being mapped in any process inside the UML. Note that I was running in 
SKAS0 mode, both host and guest are all 64-bit. Is the NX bit of the 
64-bit platform securing things off, or has SKAS3 become sort of 
obsolete?

Jan
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