[PATCH 1/3] x86: Reject bad %dr6/%dr7 values when loading guest state

[Andrew Cooper <andrew.cooper3@citrix.com>]

Right now, bad PV state is silently dropped and zeroed, while bad HVM state is
passed directly to hardware and can trigger VMEntry/VMRUN failures.  e.g.

  (XEN) d12v0 vmentry failure (reason 0x80000021): Invalid guest state (0)
  ...
  (XEN) RFLAGS=0x00000002 (0x00000002)  DR7 = 0x4000000000000001

Furthermore, prior to c/s 30f43f4aa81e ("x86: Reorganise and rename debug
register fields in struct vcpu") in Xen 4.11 where v->arch.dr6 was reduced in
width, the toolstack can cause a host crash by loading a bad %dr6 value on
VT-x hardware.

Reject any %dr6/7 values with upper bits set.  For PV guests, also audit
%dr0..3 so they aren't silently zeroed later in the function.  Leave a comment
behind explaing how %dr4/5 handling changed, and why they're ignored now.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>
CC: Jinoh Kang <jinoh.kang.kr@gmail.com>
---
 xen/arch/x86/domain.c  | 19 +++++++++++++++++++
 xen/arch/x86/hvm/hvm.c |  8 ++++++++
 2 files changed, 27 insertions(+)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index fe86a7f8530f..0698e6d486fe 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1074,8 +1074,27 @@ int arch_set_info_guest(
 #endif
     flags = c(flags);
 
+    if ( !compat )
+    {
+        if ( c(debugreg[6]) != (uint32_t)c(debugreg[6]) ||
+             c(debugreg[7]) != (uint32_t)c(debugreg[7]) )
+            return -EINVAL;
+    }
+
     if ( is_pv_domain(d) )
     {
+        /*
+         * Prior to Xen 4.11, dr5 was used to hold the emulated-only
+         * subset of dr7, and dr4 was unused.
+         *
+         * In Xen 4.11 and later, dr4/5 are written as zero, ignored for
+         * backwards compatibility, and dr7 emulation is handled
+         * internally.
+         */
+        for ( i = 0; i < ARRAY_SIZE(v->arch.dr); i++ )
+            if ( !access_ok(c(debugreg[i]), sizeof(long)) )
+                return -EINVAL;
+
         if ( !compat )
         {
             if ( !is_canonical_address(c.nat->user_regs.rip) ||
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 3a99c0ff20be..3dc2019eca67 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1032,6 +1032,14 @@ static int cf_check hvm_load_cpu_ctxt(struct domain *d, hvm_domain_context_t *h)
         return -EINVAL;
     }
 
+    if ( ctxt.dr6 != (uint32_t)ctxt.dr6 ||
+         ctxt.dr7 != (uint32_t)ctxt.dr7 )
+    {
+        printk(XENLOG_G_ERR "%pv: HVM restore: bad DR6 %#"PRIx64" or DR7 %#"PRIx64"\n",
+               v, ctxt.dr6, ctxt.dr7);
+        return -EINVAL;
+    }
+
     if ( ctxt.cr3 >> d->arch.cpuid->extd.maxphysaddr )
     {
         printk(XENLOG_G_ERR "HVM%d restore: bad CR3 %#" PRIx64 "\n",
-- 
2.30.2