[Xen-devel] [PATCH v2 1/2] xen/PVH: Set up GS segment for stack canary

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

We are making calls to C code (e.g. xen_prepare_pvh()) which may use
stack canary (stored in GS segment).

(We have to set the segment base to @canary at runtime just like
head_32.S does, from where the code fragment is taken)

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---
 arch/x86/xen/xen-pvh.S | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/xen-pvh.S b/arch/x86/xen/xen-pvh.S
index e1a5fbe..22d43eb 100644
--- a/arch/x86/xen/xen-pvh.S
+++ b/arch/x86/xen/xen-pvh.S
@@ -54,6 +54,9 @@
  * charge of setting up it's own stack, GDT and IDT.
  */
 
+#define PVH_GDT_ENTRY_CANARY   4
+#define PVH_CANARY_SEL         (PVH_GDT_ENTRY_CANARY * 8)
+
 ENTRY(pvh_start_xen)
        cld
 
@@ -64,6 +67,17 @@ ENTRY(pvh_start_xen)
        mov %eax,%es
        mov %eax,%ss
 
+       /* Set base address in stack canary descriptor. */
+       movl _pa(gdt_start),%eax
+       movl $_pa(canary),%ecx
+       movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 0(%eax)
+       shrl $16, %ecx
+       movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
+       movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 5(%eax)
+
+       mov $PVH_CANARY_SEL,%eax
+       mov %eax,%gs
+
        /* Stash hvm_start_info. */
        mov $_pa(pvh_start_info), %edi
        mov %ebx, %esi
@@ -150,9 +164,12 @@ gdt_start:
        .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* __KERNEL_CS */
 #endif
        .quad GDT_ENTRY(0xc092, 0, 0xfffff) /* __KERNEL_DS */
+       .quad GDT_ENTRY(0x4090, 0, 0x18)    /* PVH_CANARY_SEL */
 gdt_end:
 
-       .balign 4
+       .balign 16
+canary:
+       .fill 24, 1, 0
 early_stack:
        .fill 256, 1, 0
 early_stack_end:
-- 
2.9.3


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