Merge branch 'x86-mds-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 MDS mitigations from Thomas Gleixner:
 "Microarchitectural Data Sampling (MDS) is a hardware vulnerability
  which allows unprivileged speculative access to data which is
  available in various CPU internal buffers. This new set of misfeatures
  has the following CVEs assigned:

     CVE-2018-12126  MSBDS  Microarchitectural Store Buffer Data Sampling
     CVE-2018-12130  MFBDS  Microarchitectural Fill Buffer Data Sampling
     CVE-2018-12127  MLPDS  Microarchitectural Load Port Data Sampling
     CVE-2019-11091  MDSUM  Microarchitectural Data Sampling Uncacheable Memory

  MDS attacks target microarchitectural buffers which speculatively
  forward data under certain conditions. Disclosure gadgets can expose
  this data via cache side channels.

  Contrary to other speculation based vulnerabilities the MDS
  vulnerability does not allow the attacker to control the memory target
  address. As a consequence the attacks are purely sampling based, but
  as demonstrated with the TLBleed attack samples can be postprocessed
  successfully.

  The mitigation is to flush the microarchitectural buffers on return to
  user space and before entering a VM. It's bolted on the VERW
  instruction and requires a microcode update. As some of the attacks
  exploit data structures shared between hyperthreads, full protection
  requires to disable hyperthreading. The kernel does not do that by
  default to avoid breaking unattended updates.

  The mitigation set comes with documentation for administrators and a
  deeper technical view"

* 'x86-mds-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (23 commits)
  x86/speculation/mds: Fix documentation typo
  Documentation: Correct the possible MDS sysfs values
  x86/mds: Add MDSUM variant to the MDS documentation
  x86/speculation/mds: Add 'mitigations=' support for MDS
  x86/speculation/mds: Print SMT vulnerable on MSBDS with mitigations off
  x86/speculation/mds: Fix comment
  x86/speculation/mds: Add SMT warning message
  x86/speculation: Move arch_smt_update() call to after mitigation decisions
  x86/speculation/mds: Add mds=full,nosmt cmdline option
  Documentation: Add MDS vulnerability documentation
  Documentation: Move L1TF to separate directory
  x86/speculation/mds: Add mitigation mode VMWERV
  x86/speculation/mds: Add sysfs reporting for MDS
  x86/speculation/mds: Add mitigation control for MDS
  x86/speculation/mds: Conditionally clear CPU buffers on idle entry
  x86/kvm/vmx: Add MDS protection when L1D Flush is not active
  x86/speculation/mds: Clear CPU buffers on exit to user
  x86/speculation/mds: Add mds_clear_cpu_buffers()
  x86/kvm: Expose X86_FEATURE_MD_CLEAR to guests
  x86/speculation/mds: Add BUG_MSBDS_ONLY
  ...

1 files changed, 50 insertions, 0 deletions

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index daf25b60c9e3..109f974f9835 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -308,6 +308,56 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_stibp);
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+DECLARE_STATIC_KEY_FALSE(mds_user_clear);
+DECLARE_STATIC_KEY_FALSE(mds_idle_clear);
+
+#include <asm/segment.h>
+
+/**
+ * mds_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * This uses the otherwise unused and obsolete VERW instruction in
+ * combination with microcode which triggers a CPU buffer flush when the
+ * instruction is executed.
+ */
+static inline void mds_clear_cpu_buffers(void)
+{
+	static const u16 ds = __KERNEL_DS;
+
+	/*
+	 * Has to be the memory-operand variant because only that
+	 * guarantees the CPU buffer flush functionality according to
+	 * documentation. The register-operand variant does not.
+	 * Works with any segment selector, but a valid writable
+	 * data segment is the fastest variant.
+	 *
+	 * "cc" clobber is required because VERW modifies ZF.
+	 */
+	asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
+}
+
+/**
+ * mds_user_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * Clear CPU buffers if the corresponding static key is enabled
+ */
+static inline void mds_user_clear_cpu_buffers(void)
+{
+	if (static_branch_likely(&mds_user_clear))
+		mds_clear_cpu_buffers();
+}
+
+/**
+ * mds_idle_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * Clear CPU buffers if the corresponding static key is enabled
+ */
+static inline void mds_idle_clear_cpu_buffers(void)
+{
+	if (static_branch_likely(&mds_idle_clear))
+		mds_clear_cpu_buffers();
+}
+
 #endif /* __ASSEMBLY__ */
 
 /*