kvm: mmu: ITLB_MULTIHIT mitigation

commit b8e8c8303ff28c61046a4d0f6ea99aea609a7dc0 upstream.

With some Intel processors, putting the same virtual address in the TLB
as both a 4 KiB and 2 MiB page can confuse the instruction fetch unit
and cause the processor to issue a machine check resulting in a CPU lockup.

Unfortunately when EPT page tables use huge pages, it is possible for a
malicious guest to cause this situation.

Add a knob to mark huge pages as non-executable. When the nx_huge_pages
parameter is enabled (and we are using EPT), all huge pages are marked as
NX. If the guest attempts to execute in one of those pages, the page is
broken down into 4K pages, which are then marked executable.

This is not an issue for shadow paging (except nested EPT), because then
the host is in control of TLB flushes and the problematic situation cannot
happen.  With nested EPT, again the nested guest can cause problems shadow
and direct EPT is treated in the same way.

[ tglx: Fixup default to auto and massage wording a bit ]

Originally-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 4.9:
 - Use kvm_mmu_invalidate_zap_all_pages() instead of kvm_mmu_zap_all_fast()
 - Don't provide mode for nx_largepages_splitted as all stats are read-only
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 40bddc59226b..32ab3ec06fd7 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1975,6 +1975,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			KVM MMU at runtime.
 			Default is 0 (off)
 
+	kvm.nx_huge_pages=
+			[KVM] Controls the software workaround for the
+			X86_BUG_ITLB_MULTIHIT bug.
+			force	: Always deploy workaround.
+			off	: Never deploy workaround.
+			auto    : Deploy workaround based on the presence of
+				  X86_BUG_ITLB_MULTIHIT.
+
+			Default is 'auto'.
+
+			If the software workaround is enabled for the host,
+			guests do need not to enable it for nested guests.
+
 	kvm-amd.nested=	[KVM,AMD] Allow nested virtualization in KVM/SVM.
 			Default is 1 (enabled)
 
@@ -2491,6 +2504,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 					       l1tf=off [X86]
 					       mds=off [X86]
 					       tsx_async_abort=off [X86]
+					       kvm.nx_huge_pages=off [X86]
+
+				Exceptions:
+					       This does not have any effect on
+					       kvm.nx_huge_pages when
+					       kvm.nx_huge_pages=force.
 
 			auto (default)
 				Mitigate all CPU vulnerabilities, but leave SMT