[Xen-devel] [PATCH RFC 32/44] x86: Always reload the LDT on vcpu context switch

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

... and always zero the LDT for HVM contexts.  This causes erroneous execution
which manages to reference the LDT fail with a straight #GP fault, rather than
possibly finding a stale loaded LDT and wandering the #PF handler.

Future changes will cause the loading of LDT to be lazy, at which point
load_LDT() will be a nop for all cases other than context switching to/from a
PV vcpu with an LDT loaded.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/domain.c     | 5 ++---
 xen/include/asm-x86/ldt.h | 4 ++--
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index ce5337b..4671c9b 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1660,6 +1660,8 @@ static void __context_switch(void)
         lgdt(&gdt_desc);
     }
 
+    load_LDT(n);
+
     if ( pd != nd )
         cpumask_clear_cpu(cpu, pd->domain_dirty_cpumask);
     cpumask_clear_cpu(cpu, p->vcpu_dirty_cpumask);
@@ -1723,10 +1725,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
         local_irq_enable();
 
         if ( is_pv_domain(nextd) )
-        {
-            load_LDT(next);
             load_segments(next);
-        }
 
         ctxt_switch_levelling(next);
     }
diff --git a/xen/include/asm-x86/ldt.h b/xen/include/asm-x86/ldt.h
index 589daf8..6fbce93 100644
--- a/xen/include/asm-x86/ldt.h
+++ b/xen/include/asm-x86/ldt.h
@@ -7,9 +7,9 @@
 static inline void load_LDT(struct vcpu *v)
 {
     struct desc_struct *desc;
-    unsigned long ents;
+    unsigned int ents = is_pv_vcpu(v) && v->arch.pv_vcpu.ldt_ents;
 
-    if ( (ents = v->arch.pv_vcpu.ldt_ents) == 0 )
+    if ( ents == 0 )
         lldt(0);
     else
     {
-- 
2.1.4


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