x86/extable: ensure entries are swapped completely when sorting

The x86 exception table sorting was changed in commit 29934b0fb8
("x86/extable: use generic search and sort routines") to use the arch
independent code in lib/extable.c.  However, the patch was mangled
somehow on its way into the kernel from the last version posted at [1].
The committed version kind of attempted to incorporate the changes of
commit 548acf1923 ("x86/mm: Expand the exception table logic to allow
new handling options") as in _completely_ _ignoring_ the x86 specific
'handler' member of struct exception_table_entry.  This effectively
broke the sorting as entries will only partly be swapped now.

Fortunately, the x86 Kconfig selects BUILDTIME_EXTABLE_SORT, so the
exception table doesn't need to be sorted at runtime. However, in case
that ever changes, we better not break the exception table sorting just
because of that.

[ Ard Biesheuvel points out that BUILDTIME_EXTABLE_SORT applies to the
  core image only, but we still rely on the sorting routines for modules
  in that case - Linus ]

Fix this by providing a swap_ex_entry_fixup() macro that takes care of
the 'handler' member.

[1] https://lkml.org/lkml/2016/1/27/232

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Fixes: 29934b0fb8 ("x86/extable: use generic search and sort routines")
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/uaccess.h

@@ -108,6 +108,14 @@ struct exception_table_entry {
 
 #define ARCH_HAS_RELATIVE_EXTABLE
 
+#define swap_ex_entry_fixup(a, b, tmp, delta)			\
+	do {							\
+		(a)->fixup = (b)->fixup + (delta);		\
+		(b)->fixup = (tmp).fixup - (delta);		\
+		(a)->handler = (b)->handler + (delta);		\
+		(b)->handler = (tmp).handler - (delta);		\
+	} while (0)
+
 extern int fixup_exception(struct pt_regs *regs, int trapnr);
 extern bool ex_has_fault_handler(unsigned long ip);
 extern int early_fixup_exception(unsigned long *ip);