linux-kernel - [PATCH 4.14 04/48] xen/PVH: Set up GS segment for stack canary

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Date:   Fri, 27 Jul 2018 11:59:49 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Jason Andryuk <jandryuk@...il.com>
Subject: [PATCH 4.14 04/48] xen/PVH: Set up GS segment for stack canary

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Boris Ostrovsky <boris.ostrovsky@...cle.com>

commit 98014068328c5574de9a4a30b604111fd9d8f901 upstream.

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@...cle.com>
Reviewed-by: Juergen Gross <jgross@...e.com>
Signed-off-by: Juergen Gross <jgross@...e.com>
Cc: Jason Andryuk <jandryuk@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/xen/xen-pvh.S |   26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

--- 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 $_pa(canary), %eax
+	xor %edx, %edx
+	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: