Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Wed, 15 Apr 2020 16:03:29 +0200
From: Michal Suchánek <>
To: Andrew Donnellan <>
	linuxppc-dev <>
Subject: Re: CVE-2020-11669: Linux kernel 4.10 to 5.1: powerpc: guest can
 cause DoS on POWER9 KVM hosts

On Wed, Apr 15, 2020 at 10:52:53PM +1000, Andrew Donnellan wrote:
> The Linux kernel for powerpc from v4.10 to v5.1 has a bug where the
> Authority Mask Register (AMR), Authority Mask Override Register (AMOR) and
> User Authority Mask Override Register (UAMOR) are not correctly saved and
> restored when the CPU is going into/coming out of idle state.
> On POWER9 CPUs, this means that a CPU may return from idle with the AMR
> value of another thread on the same core.
> This allows a trivial Denial of Service attack against KVM hosts, by booting
> a guest kernel which makes use of the AMR, such as a v5.2 or later kernel
> with Kernel Userspace Access Prevention (KUAP) enabled.
> The guest kernel will set the AMR to prevent userspace access, then the
> thread will go idle. At a later point, the hardware thread that the guest
> was using may come out of idle and start executing in the host, without
> restoring the host AMR value. The host kernel can get caught in a page fault
> loop, as the AMR is unexpectedly causing memory accesses to fail in the
> host, and the host is eventually rendered unusable.


shouldn't the kernel restore the host registers when leaving the guest?

I recall some code exists for handling the *AM*R when leaving guest. Can
the KVM guest enter idle without exiting to host?



Powered by blists - more mailing lists

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.