x86/speculation/mds: Add BUG_MSBDS_ONLY

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commit e261f209c3 upstream.

This bug bit is set on CPUs which are only affected by Microarchitectural
Store Buffer Data Sampling (MSBDS) and not by any other MDS variant.

This is important because the Store Buffers are partitioned between
Hyper-Threads so cross thread forwarding is not possible. But if a thread
enters or exits a sleep state the store buffer is repartitioned which can
expose data from one thread to the other. This transition can be mitigated.

That means that for CPUs which are only affected by MSBDS SMT can be
enabled, if the CPU is not affected by other SMT sensitive vulnerabilities,
e.g. L1TF. The XEON PHI variants fall into that category. Also the
Silvermont/Airmont ATOMs, but for them it's not really relevant as they do
not support SMT, but mark them for completeness sake.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>
[bwh: Backported to 3.16:
 - Assign the next available bug flag
 - Adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

...

This commit is contained in:

Thomas Gleixner 2019-03-01 20:21:08 +01:00 • committed by Ben Hutchings

parent 78e6f63195

commit 8a73fd7fa6

2 changed files with 13 additions and 8 deletions

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1

arch/x86/include/asm/cpufeatures.h

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20

arch/x86/kernel/cpu/common.c

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