linux-kernel - Re: Sanitize CPU-state when switching tasks (was sanitize CPU-state when switching from virtual-8086 mode to other task)

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Date:	Wed, 08 Jan 2014 07:45:50 +0000
From:	halfdog <me@...fdog.net>
To:	"H. Peter Anvin" <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Ben Hutchings <ben@...adent.org.uk>
Subject: Re: Sanitize CPU-state when switching tasks (was sanitize CPU-state
 when switching from virtual-8086 mode to other task)

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Update to the issue:

* Although first observed with virtual-8086 mode, the bug is not
specific to virtual-8086 mode, it can be triggered with normal x86
userspace code also, even with better reproducibility.

* Ben Hutchings looked at the Debian bug report [1], he failed to
reproduce on his hardware, so it might be specific to some CPU models
(currently my AMD E-350 is only machine known to be affected).

* When deactivating mmap_min_addr, the NULL-dereferences during
task-switch is exploitable, works both on native hardware and within
VirtualBox. See [2] for POC to gain root privileges.

* It seems, that when changing the FPU control word with "fstcw" just
before exit of the process, then another process could suffer when
doing __do_switch, probably related to the xsave instruction and a x86
processor bug workaround, see "noxsave" switch [3]: [BUGS=X86]
Disables x86 extended register state save and restore using xsave. The
kernel will fallback to enabling legacy floating-point and sse state.

hd

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=733551
[2]
http://www.halfdog.net/Security/2013/Vm86SyscallTaskSwitchKernelPanic/
[3] https://www.kernel.org/doc/Documentation/kernel-parameters.txt

- -- 
http://www.halfdog.net/
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