x86/xen: fix pv boot

Commit 9da3f2b7405440 ("x86/fault: BUG() when uaccess helpers fault on
kernel addresses") introduced a regression for booting Xen PV guests.

Xen PV guests are using __put_user() and __get_user() for accessing the
p2m map (physical to machine frame number map) as accesses might fail
in case of not populated areas of the map.

With above commit using __put_user() and __get_user() for accessing
kernel pages is no longer valid. So replace the Xen hack by adding
appropriate p2m access functions using the default fixup handler.

Fixes: 9da3f2b7405440 ("x86/fault: BUG() when uaccess helpers fault on kernel addresses")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/include/asm/xen/page.h

@@ -9,7 +9,7 @@
 #include <linux/mm.h>
 #include <linux/device.h>
 
-#include <linux/uaccess.h>
+#include <asm/extable.h>
 #include <asm/page.h>
 #include <asm/pgtable.h>
 
@@ -93,12 +93,39 @@ clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
  */
 static inline int xen_safe_write_ulong(unsigned long *addr, unsigned long val)
 {
-	return __put_user(val, (unsigned long __user *)addr);
+	int ret = 0;
+
+	asm volatile("1: mov %[val], %[ptr]\n"
+		     "2:\n"
+		     ".section .fixup, \"ax\"\n"
+		     "3: sub $1, %[ret]\n"
+		     "   jmp 2b\n"
+		     ".previous\n"
+		     _ASM_EXTABLE(1b, 3b)
+		     : [ret] "+r" (ret), [ptr] "=m" (*addr)
+		     : [val] "r" (val));
+
+	return ret;
 }
 
-static inline int xen_safe_read_ulong(unsigned long *addr, unsigned long *val)
+static inline int xen_safe_read_ulong(const unsigned long *addr,
+				      unsigned long *val)
 {
-	return __get_user(*val, (unsigned long __user *)addr);
+	int ret = 0;
+	unsigned long rval = ~0ul;
+
+	asm volatile("1: mov %[ptr], %[rval]\n"
+		     "2:\n"
+		     ".section .fixup, \"ax\"\n"
+		     "3: sub $1, %[ret]\n"
+		     "   jmp 2b\n"
+		     ".previous\n"
+		     _ASM_EXTABLE(1b, 3b)
+		     : [ret] "+r" (ret), [rval] "+r" (rval)
+		     : [ptr] "m" (*addr));
+	*val = rval;
+
+	return ret;
 }
 
 #ifdef CONFIG_XEN_PV

arch/x86/xen/p2m.c

@@ -656,8 +656,7 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned long mfn)
 
 	/*
 	 * The interface requires atomic updates on p2m elements.
-	 * xen_safe_write_ulong() is using __put_user which does an atomic
-	 * store via asm().
+	 * xen_safe_write_ulong() is using an atomic store via asm().
 	 */
 	if (likely(!xen_safe_write_ulong(xen_p2m_addr + pfn, mfn)))
 		return true;