x86/32on64: don't modify guest descriptors without need

System gates with type 0 shouldn't have what might be their DPL altered
- such descriptors can't be used anyway without incurring a #GP, and
hence adjusting its DPL is only risking to confuse the guest.

Also bail right away for non-present descriptors - no need to write
back anything in that case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index 250d3e002567d317b5f097d2f51a298fe861f337..b8b6b70a06d774ac67e976409fa3f81660db8e8e 100644 (file)
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -1088,7 +1088,7 @@ int check_descriptor(const struct domain *dom, struct desc_struct *d)
 
     /* A not-present descriptor will always fault, so is safe. */
     if ( !(b & _SEGMENT_P) ) 
-        goto good;
+        return 1;
 
     /* Check and fix up the DPL. */
     dpl = (b >> 13) & 3;
@@ -1130,7 +1130,7 @@ int check_descriptor(const struct domain *dom, struct desc_struct *d)
 
     /* Invalid type 0 is harmless. It is used for 2nd half of a call gate. */
     if ( (b & _SEGMENT_TYPE) == 0x000 )
-        goto good;
+        return 1;
 
     /* Everything but a call gate is discarded here. */
     if ( (b & _SEGMENT_TYPE) != 0xc00 )