CVE-2019-19338 Kernel: KVM: export MSR_IA32_TSX_CTRL to guest - incomplete fix for TAA (CVE-2019-11135)

[P J P <ppandit () redhat com>]

  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

'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