From: "Michal Suchánek" <msuchanek@suse•de>
To: Andrew Donnellan <ajd@linux•ibm.com>
Cc: oss-security@lists•openwall.com,
linuxppc-dev <linuxppc-dev@lists•ozlabs.org>
Subject: Re: CVE-2020-11669: Linux kernel 4.10 to 5.1: powerpc: guest can cause DoS on POWER9 KVM hosts
Date: Wed, 15 Apr 2020 16:03:29 +0200 [thread overview]
Message-ID: <20200415140329.GC25468@kitsune.suse.cz> (raw)
In-Reply-To: <2ff92392-30ec-d5c4-84c9-e6ba24f6b154@linux.ibm.com>
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.
Hello,
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?
Thanks
Michal
next prev parent reply other threads:[~2020-04-15 14:05 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-15 12:52 CVE-2020-11669: Linux kernel 4.10 to 5.1: powerpc: guest can cause DoS on POWER9 KVM hosts Andrew Donnellan
2020-04-15 14:03 ` Michal Suchánek [this message]
2020-04-16 4:02 ` Paul Mackerras
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