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@suse.com>
Signed-off-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Keir Fraser <keir.xen@gmail.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>
xen-unstable changeset:   25480:76eaf5966c05
xen-unstable date:        Tue Jun 12 11:33:40 2012 +0100
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index e6483f15dbd1ebbf1601252d708fd49f3795fc8e..f42630f2dc5ff4c5672e2d1b21e9124e0761ff63 100644 (file)
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -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: