x86/xen: init %gs very early to avoid page faults with stack protector

commit 4f277295e54c5b7340e48efea3fc5cc21a2872b7 upstream.

When running as Xen pv guest %gs is initialized some time after
C code is started. Depending on stack protector usage this might be
too late, resulting in page faults.

So setup %gs and MSR_GS_BASE in assembly code already.

Cc: stable@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: Chris Patterson <cjp256@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 497cc55..96f26e0 100644 (file)
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -9,7 +9,9 @@
 
 #include <asm/boot.h>
 #include <asm/asm.h>
+#include <asm/msr.h>
 #include <asm/page_types.h>
+#include <asm/percpu.h>
 #include <asm/unwind_hints.h>
 
 #include <xen/interface/elfnote.h>
@@ -35,6 +37,20 @@ ENTRY(startup_xen)
        mov %_ASM_SI, xen_start_info
        mov $init_thread_union+THREAD_SIZE, %_ASM_SP
 
+#ifdef CONFIG_X86_64
+       /* Set up %gs.
+        *
+        * The base of %gs always points to the bottom of the irqstack
+        * union.  If the stack protector canary is enabled, it is
+        * located at %gs:40.  Note that, on SMP, the boot cpu uses
+        * init data section till per cpu areas are set up.
+        */
+       movl    $MSR_GS_BASE,%ecx
+       movq    $INIT_PER_CPU_VAR(irq_stack_union),%rax
+       cdq
+       wrmsr
+#endif
+
        jmp xen_start_kernel
 END(startup_xen)
        __FINIT