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Date: Tue, 10 Dec 2019 12:09:22 -0600
From: Tyler Hicks <tyhicks@...onical.com>
To: oss-security@...ts.openwall.com
Subject: Re: CVE-2019-19338 Kernel: KVM: export
 MSR_IA32_TSX_CTRL to guest - incomplete fix for TAA (CVE-2019-11135)

On 2019-12-10 17:10:40, P J P wrote:
>   Hello,

Hello!

> Transaction Asynchronous Abort (TAA) h/w issue, which affects Intel CPUs, is
> mitigated in two ways. One is by disabling Transactional Synchronisation
> Extensions (TSX) feature of the CPU. And second is by clearing the affected
> Store/Fill/Load port architectural buffers, which may hold sensitive
> information bits.
> 
> It was found that the current kernel fixes don't completely fix TAA issue
> for guest VMs. When a guest is running on a host CPU affected by TAA (ie.
> TAA_NO=0) but not affected by MDS issue (ie MDS_NO=1), to mitigate TAA
> issue, guest was to clear the affected buffers by using VERW instruction
> mechanism. But when MDS_NO=1 bit was exported to the guests, it did not
> quite use the VERW mechanism to clear the affected buffers.
> 
> This issue affects guests running on Cascade Lake CPUs, which are affected
> by the TAA (ie. TAA_NO=0) issue, but are not affected by the MDS (ie.
> MDS_NO=1) issue.
> 
> It requires that host has 'TSX' enabled.
> 
> Upstream patches:
> -----------------
>   -> https://git.kernel.org/linus/cbbaa2727aa3ae9e0a844803da7cef7fd3b94f2b
>   -> https://git.kernel.org/linus/c11f83e0626bdc2b6c550fc8b9b6eeefbd8cefaa
>   -> https://git.kernel.org/linus/b07a5c53d42a8c87b208614129e947dd2338ff9c
> 
> Another option: Export MDS_NO=0 to guests when TSX is enabled
>   -> https://git.kernel.org/linus/e1d38b63acd843cfdd4222bf19a26700fd5c699e

If this commit, which was part of the initial set of mitigation patches
for TAA, is a valid option then I'm not understanding how there was an
incomplete fix.

Is the CVE assignment specifically for a distro kernel that didn't
include commit e1d38b63acd8 ("kvm/x86: Export MDS_NO=0 to guests when
TSX is enabled")?

Tyler

> 
> 'CVE-2019-19338' is assigned by Red Hat Inc.
> 
> Thank you.
> --
> Prasad J Pandit / Red Hat Product Security Team
> 8685 545E B54C 486B C6EB 271E E285 8B5A F050 DE8D
> 

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