[Xen-devel] [PATCH v4 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).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---
 arch/x86/xen/xen-pvh.S | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/xen-pvh.S b/arch/x86/xen/xen-pvh.S
index e1a5fbe..0169374 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
 
@@ -98,6 +101,12 @@ ENTRY(pvh_start_xen)
        /* 64-bit entry point. */
        .code64
 1:
+       /* Set base address in stack canary descriptor. */
+       mov $MSR_GS_BASE,%ecx
+       mov $canary, %rax
+       cdq
+       wrmsr
+
        call xen_prepare_pvh
 
        /* startup_64 expects boot_params in %rsi. */
@@ -107,6 +116,17 @@ ENTRY(pvh_start_xen)
 
 #else /* CONFIG_X86_64 */
 
+       /* Set base address in stack canary descriptor. */
+       movl $_pa(gdt_start),%eax
+       movl $_pa(canary),%ecx
+       movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax)
+       shrl $16, %ecx
+       movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax)
+       movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax)
+
+       mov $PVH_CANARY_SEL,%eax
+       mov %eax,%gs
+
        call mk_early_pgtbl_32
 
        mov $_pa(initial_page_table), %eax
@@ -150,9 +170,13 @@ 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 48, 1, 0
+
 early_stack:
        .fill 256, 1, 0
 early_stack_end:
-- 
2.9.3


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