linux-kernel - [PATCH 4.18 58/79] x86/KVM/VMX: Initialize the vmx_l1d_flush

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Date:   Tue, 14 Aug 2018 19:17:17 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 4.18 58/79] x86/KVM/VMX: Initialize the vmx_l1d_flush_pages content

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nicolai Stange <nstange@...e.de>

The slow path in vmx_l1d_flush() reads from vmx_l1d_flush_pages in order
to evict the L1d cache.

However, these pages are never cleared and, in theory, their data could be
leaked.

More importantly, KSM could merge a nested hypervisor's vmx_l1d_flush_pages
to fewer than 1 << L1D_CACHE_ORDER host physical pages and this would break
the L1d flushing algorithm: L1D on x86_64 is tagged by physical addresses.

Fix this by initializing the individual vmx_l1d_flush_pages with a
different pattern each.

Rename the "empty_zp" asm constraint identifier in vmx_l1d_flush() to
"flush_pages" to reflect this change.

Fixes: a47dd5f06714 ("x86/KVM/VMX: Add L1D flush algorithm")
Signed-off-by: Nicolai Stange <nstange@...e.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/kvm/vmx.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -211,6 +211,7 @@ static void *vmx_l1d_flush_pages;
 static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
 {
 	struct page *page;
+	unsigned int i;
 
 	if (!enable_ept) {
 		l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_EPT_DISABLED;
@@ -243,6 +244,16 @@ static int vmx_setup_l1d_flush(enum vmx_
 		if (!page)
 			return -ENOMEM;
 		vmx_l1d_flush_pages = page_address(page);
+
+		/*
+		 * Initialize each page with a different pattern in
+		 * order to protect against KSM in the nested
+		 * virtualization case.
+		 */
+		for (i = 0; i < 1u << L1D_CACHE_ORDER; ++i) {
+			memset(vmx_l1d_flush_pages + i * PAGE_SIZE, i + 1,
+			       PAGE_SIZE);
+		}
 	}
 
 	l1tf_vmx_mitigation = l1tf;
@@ -9737,7 +9748,7 @@ static void vmx_l1d_flush(struct kvm_vcp
 		/* First ensure the pages are in the TLB */
 		"xorl	%%eax, %%eax\n"
 		".Lpopulate_tlb:\n\t"
-		"movzbl	(%[empty_zp], %%" _ASM_AX "), %%ecx\n\t"
+		"movzbl	(%[flush_pages], %%" _ASM_AX "), %%ecx\n\t"
 		"addl	$4096, %%eax\n\t"
 		"cmpl	%%eax, %[size]\n\t"
 		"jne	.Lpopulate_tlb\n\t"
@@ -9746,12 +9757,12 @@ static void vmx_l1d_flush(struct kvm_vcp
 		/* Now fill the cache */
 		"xorl	%%eax, %%eax\n"
 		".Lfill_cache:\n"
-		"movzbl	(%[empty_zp], %%" _ASM_AX "), %%ecx\n\t"
+		"movzbl	(%[flush_pages], %%" _ASM_AX "), %%ecx\n\t"
 		"addl	$64, %%eax\n\t"
 		"cmpl	%%eax, %[size]\n\t"
 		"jne	.Lfill_cache\n\t"
 		"lfence\n"
-		:: [empty_zp] "r" (vmx_l1d_flush_pages),
+		:: [flush_pages] "r" (vmx_l1d_flush_pages),
 		    [size] "r" (size)
 		: "eax", "ebx", "ecx", "edx");
 }