[PATCH 2/7] x86/traps: Poison unused stack pointers in the TSS

[Andrew Cooper <andrew.cooper3@citrix.com>]

This is for additional defence-in-depth following LDT/GDT/IDT corruption.

It causes attempted control transfers to ring 1 or 2 (via a call gate), or
attempts to use IST 3 through 7 to yield #SS[0], rather than executing with a
stack starting at the top of virtual address space.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
---
 xen/arch/x86/cpu/common.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 6c27008..8796568 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -645,6 +645,14 @@ void load_system_tables(void)
 	tss->ist[IST_DF  - 1] = stack_top + IST_DF  * PAGE_SIZE;
 	tss->ist[IST_NMI - 1] = stack_top + IST_NMI * PAGE_SIZE;
 
+	/* Poision all other stack pointers to prevent their accidental use. */
+	tss->rsp1   = 0x8600111111111111ul;
+	tss->rsp2   = 0x8600222222222222ul;
+	tss->ist[3] = 0x8600444444444444ul;
+	tss->ist[4] = 0x8600555555555555ul;
+	tss->ist[5] = 0x8600666666666666ul;
+	tss->ist[6] = 0x8600777777777777ul;
+
 	_set_tssldt_desc(
 		gdt + TSS_ENTRY,
 		(unsigned long)tss,
-- 
2.1.4


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