[PATCH v3 2/3] x86: relax LDT check in arch_set_info_guest()

[Jan Beulich <jbeulich@xxxxxxxx>]

It is wrong for us to check the base address when there's no LDT in the
first place. Once we don't do this check anymore we can also set the
base address to a non-canonical value when the LDT is empty.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
v3: Re-base over changes to earlier patch.
v2: Set v->arch.pv.ldt_base to non-canonical for an empty LDT, plus
    related necessary adjustments.

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -967,8 +967,10 @@ int arch_set_info_guest(
         if ( !compat && !(flags & VGCF_in_kernel) && !c.nat->ctrlreg[1] )
             return -EINVAL;
 
-        v->arch.pv.ldt_base = c(ldt_base);
         v->arch.pv.ldt_ents = c(ldt_ents);
+        v->arch.pv.ldt_base = v->arch.pv.ldt_ents
+                              ? c(ldt_base)
+                              : (unsigned long)ZERO_BLOCK_PTR;
     }
     else
     {
@@ -997,8 +999,9 @@ int arch_set_info_guest(
         for ( i = 0; !fail && i < nr_gdt_frames; ++i )
             fail = v->arch.pv.gdt_frames[i] != c(gdt_frames[i]);
 
-        fail |= v->arch.pv.ldt_base != c(ldt_base);
         fail |= v->arch.pv.ldt_ents != c(ldt_ents);
+        if ( v->arch.pv.ldt_ents )
+            fail |= v->arch.pv.ldt_base != c(ldt_base);
 
         if ( fail )
            return -EOPNOTSUPP;
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -1583,7 +1583,7 @@ void arch_get_info_guest(struct vcpu *v,
     }
     else
     {
-        c(ldt_base = v->arch.pv.ldt_base);
+        c(ldt_base = v->arch.pv.ldt_ents ? v->arch.pv.ldt_base : 0);
         c(ldt_ents = v->arch.pv.ldt_ents);
         for ( i = 0; i < ARRAY_SIZE(v->arch.pv.gdt_frames); ++i )
             c(gdt_frames[i] = v->arch.pv.gdt_frames[i]);
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3669,14 +3669,15 @@ long do_mmuext_op(
         case MMUEXT_SET_LDT:
         {
             unsigned int ents = op.arg2.nr_ents;
-            unsigned long ptr = ents ? op.arg1.linear_addr : 0;
+            unsigned long ptr = ents ? op.arg1.linear_addr
+                                     : (unsigned long)ZERO_BLOCK_PTR;
 
             if ( unlikely(currd != pg_owner) )
                 rc = -EPERM;
             else if ( paging_mode_external(currd) )
                 rc = -EINVAL;
-            else if ( ((ptr & (PAGE_SIZE - 1)) != 0) || !__addr_ok(ptr) ||
-                      (ents > 8192) )
+            else if ( (ents > 8192) ||
+                      (ents && ((ptr & (PAGE_SIZE - 1)) || !__addr_ok(ptr))) )
             {
                 gdprintk(XENLOG_WARNING,
                          "Bad args to SET_LDT: ptr=%lx, ents=%x\n", ptr, ents);