From: Jan Beulich <jbeulich@suse.com>
Date: Wed, 23 Nov 2016 14:25:35 +0000 (+0100)
Subject: x86/HVM: limit writes to incoming TSS during task switch
X-Git-Tag: 4.8.0-rc7~15
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=db153db08846d038a6fd23fee7719dfee5d27ffc;p=xen.git

x86/HVM: limit writes to incoming TSS during task switch

The only field modified (and even that conditionally) is the back link.
Write only that field, and only when it actually has been written to.

Take the opportunity and also ditch the pointless initializer from the
"tss" local variable, which gets completely filled anyway by reading
from guest memory.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
---

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 1f9666a533..bde7640b1d 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2890,7 +2890,7 @@ void hvm_task_switch(
         u32 cr3, eip, eflags, eax, ecx, edx, ebx, esp, ebp, esi, edi;
         u16 es, _3, cs, _4, ss, _5, ds, _6, fs, _7, gs, _8, ldt, _9;
         u16 trace, iomap;
-    } tss = { 0 };
+    } tss;
 
     hvm_get_segment_register(v, x86_seg_gdtr, &gdt);
     hvm_get_segment_register(v, x86_seg_tr, &prev_tr);
@@ -3010,12 +3010,6 @@ void hvm_task_switch(
     regs->esi    = tss.esi;
     regs->edi    = tss.edi;
 
-    if ( (taskswitch_reason == TSW_call_or_int) )
-    {
-        regs->eflags |= X86_EFLAGS_NT;
-        tss.back_link = prev_tr.sel;
-    }
-
     exn_raised = 0;
     if ( hvm_load_segment_selector(x86_seg_es, tss.es, tss.eflags) ||
          hvm_load_segment_selector(x86_seg_cs, tss.cs, tss.eflags) ||
@@ -3025,12 +3019,18 @@ void hvm_task_switch(
          hvm_load_segment_selector(x86_seg_gs, tss.gs, tss.eflags) )
         exn_raised = 1;
 
-    rc = hvm_copy_to_guest_virt(
-        tr.base, &tss, sizeof(tss), PFEC_page_present);
-    if ( rc == HVMCOPY_bad_gva_to_gfn )
-        exn_raised = 1;
-    else if ( rc != HVMCOPY_okay )
-        goto out;
+    if ( taskswitch_reason == TSW_call_or_int )
+    {
+        regs->eflags |= X86_EFLAGS_NT;
+        tss.back_link = prev_tr.sel;
+
+        rc = hvm_copy_to_guest_virt(tr.base + offsetof(typeof(tss), back_link),
+                                    &tss.back_link, sizeof(tss.back_link), 0);
+        if ( rc == HVMCOPY_bad_gva_to_gfn )
+            exn_raised = 1;
+        else if ( rc != HVMCOPY_okay )
+            goto out;
+    }
 
     if ( (tss.trace & 1) && !exn_raised )
         hvm_inject_hw_exception(TRAP_debug, HVM_DELIVER_NO_ERROR_CODE);