CVE-2020-11669: Linux kernel 4.10 to 5.1: powerpc: guest can cause DoS on POWER9 KVM hosts

[Michal Suchánek]

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