[Xen-devel] [PATCH v2] x86/32on64: don't modify guest descriptors without need

["Jan Beulich" <JBeulich@xxxxxxxx>]

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@xxxxxxxx>
---
v2: Broken out from a larger patch.

--- 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
 
     /* 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
 
     /* 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 )




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