[Xen-changelog] [xen-4.0-testing] x86_64: Do not execute sysret with a non-canonical return address

[Xen patchbot-4.0-testing <patchbot@xxxxxxx>]

# HG changeset patch
# User Jan Beulich <JBeulich@xxxxxxxx>
# Date 1339497511 -3600
# Node ID dd367837e08919f10a57b7fa482e68a91e0779dc
# Parent  c6eb61ed6f04b4079525c3944b5a55268e1db4f1
x86_64: Do not execute sysret with a non-canonical return address

Check for non-canonical guest RIP before attempting to execute sysret.
If sysret is executed with a non-canonical value in RCX, Intel CPUs
take the fault in ring0, but we will necessarily already have switched
to the the user's stack pointer.

This is a security vulnerability, XSA-7 / CVE-2012-0217.

Signed-off-by: Jan Beulich <JBeulich@xxxxxxxx>
Signed-off-by: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
Acked-by: Keir Fraser <keir.xen@xxxxxxxxx>
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

xen-unstable changeset:   25480:76eaf5966c05
xen-unstable date:        Tue Jun 12 11:33:40 2012 +0100
Committed-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---


diff -r c6eb61ed6f04 -r dd367837e089 xen/arch/x86/x86_64/entry.S
--- a/xen/arch/x86/x86_64/entry.S       Mon May 14 17:02:16 2012 +0100
+++ b/xen/arch/x86/x86_64/entry.S       Tue Jun 12 11:38:31 2012 +0100
@@ -51,6 +51,13 @@ restore_all_guest:
         testw $TRAP_syscall,4(%rsp)
         jz    iret_exit_to_guest
 
+        /* Don't use SYSRET path if the return address is not canonical. */
+        movq  8(%rsp),%rcx
+        sarq  $47,%rcx
+        incl  %ecx
+        cmpl  $1,%ecx
+        ja    .Lforce_iret
+
         addq  $8,%rsp
         popq  %rcx                    # RIP
         popq  %r11                    # CS
@@ -61,6 +68,10 @@ restore_all_guest:
         sysretq
 1:      sysretl
 
+.Lforce_iret:
+        /* Mimic SYSRET behavior. */
+        movq  8(%rsp),%rcx            # RIP
+        movq  24(%rsp),%r11           # RFLAGS
         ALIGN
 /* No special register assumptions. */
 iret_exit_to_guest:

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