[Xen-devel] [PATCH 1/2] x86: don't latch wrong (stale) GS base addresses

["Jan Beulich" <JBeulich@xxxxxxxx>]

load_segments() writes selector registers before doing any of the base
address updates. Any of these selector loads can cause a page fault in
case it references the LDT, and the LDT page accessed was only recently
installed. Therefore the call tree map_ldt_shadow_page() ->
guest_get_eff_kern_l1e() -> toggle_guest_mode() would in such a case
wrongly latch the outgoing vCPU's GS.base into the incoming vCPU's
recorded state.

Split page table toggling from GS handling - neither
guest_get_eff_kern_l1e() nor guest_io_okay() need more than the page
tables being the kernel ones for the memory access they want to do.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -233,8 +233,17 @@ void toggle_guest_mode(struct vcpu *v)
         else
             v->arch.pv_vcpu.gs_base_user = __rdgsbase();
     }
-    v->arch.flags ^= TF_kernel_mode;
     asm volatile ( "swapgs" );
+
+    toggle_guest_pt(v);
+}
+
+void toggle_guest_pt(struct vcpu *v)
+{
+    if ( is_pv_32bit_vcpu(v) )
+        return;
+
+    v->arch.flags ^= TF_kernel_mode;
     update_cr3(v);
     /* Don't flush user global mappings from the TLB. Don't tick TLB clock. */
     asm volatile ( "mov %0, %%cr3" : : "r" (v->arch.cr3) : "memory" );
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -137,7 +137,7 @@ static bool guest_io_okay(unsigned int p
          * read as 0xff (no access allowed).
          */
         if ( user_mode )
-            toggle_guest_mode(v);
+            toggle_guest_pt(v);
 
         switch ( __copy_from_guest_offset(x.bytes, v->arch.pv_vcpu.iobmp,
                                           port>>3, 2) )
@@ -150,7 +150,7 @@ static bool guest_io_okay(unsigned int p
         }
 
         if ( user_mode )
-            toggle_guest_mode(v);
+            toggle_guest_pt(v);
 
         if ( (x.mask & (((1 << bytes) - 1) << (port & 7))) == 0 )
             return true;
--- a/xen/arch/x86/pv/mm.c
+++ b/xen/arch/x86/pv/mm.c
@@ -72,12 +72,12 @@ static l1_pgentry_t guest_get_eff_kern_l
     l1_pgentry_t l1e;
 
     if ( user_mode )
-        toggle_guest_mode(curr);
+        toggle_guest_pt(curr);
 
     l1e = guest_get_eff_l1e(linear);
 
     if ( user_mode )
-        toggle_guest_mode(curr);
+        toggle_guest_pt(curr);
 
     return l1e;
 }
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -76,6 +76,8 @@ void mapcache_override_current(struct vc
 
 /* x86/64: toggle guest between kernel and user modes. */
 void toggle_guest_mode(struct vcpu *);
+/* x86/64: toggle guest page tables between kernel and user modes. */
+void toggle_guest_pt(struct vcpu *);
 
 /*
  * Initialise a hypercall-transfer page. The given pointer must be mapped




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