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[PATCH v5 1/4] x86/xen: Unmap aliases in xen_alloc_ldt and xen_free_ldt

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  [PATCH v5 1/4] x86/xen: Unmap aliases in xen_alloc_ldt and xen_free_ldt Andy Lutomirski <luto@kernel.org> - 2015-07-28 07:30 +0200

#1193765 — [PATCH v5 1/4] x86/xen: Unmap aliases in xen_alloc_ldt and xen_free_ldt

I've been able to get an unmodified Xen guest to OOPS once after a
lot of test iterations without this patch.  I think this patch fixes
the problem.  I'm a bit surprised that we don't see much more severe
LDT problems on Xen without this fix.

Once the synchronous modify_ldt code causes modify_ldt to more
aggressively reallocate the LDT, the OOPSes become much more common
without this fix.

Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/xen/enlighten.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 0b95c9b8283f..e417d08c56c4 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;
 
+	/*
+	 * The set_aliased_prot call may OOPS due to a hypercall failure
+	 * if there are any lazy vmap aliases of the page.  We don't
+	 * need to call vm_unmap_aliases() here, though, because we
+	 * already eliminated any aliases in xen_alloc_ldt.
+	 */
+
 	for(i = 0; i < entries; i += entries_per_page)
 		set_aliased_prot(ldt + i, PAGE_KERNEL);
 }
-- 
2.4.3

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