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Subject[PATCH] x86/xen: fix pv boot
Date
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kernel pages is no longer valid.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>
---
arch/x86/include/asm/xen/page.h | 39 ++++++++++++++++++++++++++++++++++++---
arch/x86/xen/p2m.c | 3 +--
2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index 123e669bf363..a39015cb2f3f 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/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>

@@ -91,15 +91,48 @@ clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
* Helper functions to write or read unsigned long values to/from
* memory, when the access may fault.
*/
+#ifdef CONFIG_X86_32
+#define __i "l"
+#define __r "k"
+#else
+#define __i "q"
+#define __r ""
+#endif
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"__i" %"__r"1,%2\n"
+ "2:\n"
+ ".section .fixup,\"ax\"\n"
+ "3: mov %3,%0\n"
+ " jmp 2b\n"
+ ".previous\n"
+ _ASM_EXTABLE(1b, 3b)
+ : "=r"(ret)
+ : "r" (val), "m" (*addr), "i" (-1), "0" (ret));
+ return ret;
}

static inline int xen_safe_read_ulong(unsigned long *addr, unsigned long *val)
{
- return __get_user(*val, (unsigned long __user *)addr);
+ int ret = 0;
+ unsigned long rval;
+
+ asm volatile("1: mov"__i" %2,%"__r"1\n"
+ "2:\n"
+ ".section .fixup,\"ax\"\n"
+ "3: mov %3,%0\n"
+ " jmp 2b\n"
+ ".previous\n"
+ _ASM_EXTABLE(1b, 3b)
+ : "=r" (ret), "=r" (rval)
+ : "m" (*addr), "i" (-1), "0" (ret));
+ *val = rval;
+ return ret;
}
+#undef __i
+#undef __r

#ifdef CONFIG_XEN_PV
/*
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index b06731705529..055e37e43541 100644
--- a/arch/x86/xen/p2m.c
+++ b/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;
--
2.16.4
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