commit b4772cf849f05d5ceab079b4e88497dd1b990acd
Author: Andy Lutomirski <luto@xxxxxxxxxx>
Date:   Mon Jul 27 20:20:55 2015 -0700

    x86/xen: Unmap aliases in xen_alloc_ldt and xen_free_ldt
    
    The xen_free_ldt change fixes an OOPS in the new modify_ldt
    implementation.  I think the xen_alloc_ldt change should be
    necessary, too, but I can't seem to trigger any failures without it,
    which I find surprising.
    
    Cc: stable@xxxxxxxxxxxxxxx
    Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 0b95c9b8283f..100a2e2294af 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -32,6 +32,7 @@
 #include <linux/gfp.h>
 #include <linux/memblock.h>
 #include <linux/edd.h>
+#include <linux/vmalloc.h>
 
 #include <xen/xen.h>
 #include <xen/events.h>
@@ -512,6 +513,10 @@ static void xen_alloc_ldt(struct desc_struct *ldt, unsigned entries)
 
 	for(i = 0; i < entries; i += entries_per_page)
 		set_aliased_prot(ldt + i, PAGE_KERNEL_RO);
+
+	/* If there are stray aliases, the LDT won't work. */
+	if (is_vmalloc_addr(ldt))
+		vm_unmap_aliases();
 }
 
 static void xen_free_ldt(struct desc_struct *ldt, unsigned entries)
@@ -519,6 +524,13 @@ static void xen_free_ldt(struct desc_struct *ldt, unsigned entries)
 	const unsigned entries_per_page = PAGE_SIZE / LDT_ENTRY_SIZE;
 	int i;
 
+	/*
+	 * If there are stray aliases, hypervisor will fail the hypercalls,
+	 * causing us to OOPS.
+	 */
+	if (is_vmalloc_addr(ldt))
+		vm_unmap_aliases();
+
 	for(i = 0; i < entries; i += entries_per_page)
 		set_aliased_prot(ldt + i, PAGE_KERNEL);
 }
diff --git a/tools/testing/selftests/x86/ldt_gdt.c b/tools/testing/selftests/x86/ldt_gdt.c
index c27adfc9ae72..fba5bc133aa2 100644
--- a/tools/testing/selftests/x86/ldt_gdt.c
+++ b/tools/testing/selftests/x86/ldt_gdt.c
@@ -204,6 +204,10 @@ static void do_simple_tests(void)
 	};
 	install_valid(&desc, AR_DPL3 | AR_TYPE_XRCODE | AR_S | AR_P | AR_DB);
 
+	desc.entry_number = 8191;
+	install_valid(&desc, AR_DPL3 | AR_TYPE_XRCODE | AR_S | AR_P | AR_DB);
+	desc.entry_number = 0;
+
 	desc.limit_in_pages = 1;
 	install_valid(&desc, AR_DPL3 | AR_TYPE_XRCODE |
 		      AR_S | AR_P | AR_DB | AR_G);