x86: limit checks in hypercall_xlat_continuation() to actual arguments

HVM/PVH guests can otherwise trigger the final BUG_ON() in that
function by entering 64-bit mode, setting the high halves of affected
registers to non-zero values, leaving 64-bit mode, and issuing a
hypercall that might get preempted and hence become subject to
continuation argument translation (HYPERVISOR_memory_op being the only
one possible for HVM, PVH also having the option of using
HYPERVISOR_mmuext_op). This issue got introduced when HVM code was
switched to use compat_memory_op() - neither that nor
hypercall_xlat_continuation() were originally intended to be used by
other than PV guests (which can't enter 64-bit mode and hence have no
way to alter the high halves of 64-bit registers).

This is CVE-2014-8866 / XSA-111.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 73d01bbd0186e087677944aacf3a87bbc9c254d9..11c7d9fdb7fa55af55e7d43a7110ff62f21e7e04 100644 (file)
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1750,7 +1750,8 @@ unsigned long hypercall_create_continuation(
     return op;
 }
 
-int hypercall_xlat_continuation(unsigned int *id, unsigned int mask, ...)
+int hypercall_xlat_continuation(unsigned int *id, unsigned int nr,
+                                unsigned int mask, ...)
 {
     int rc = 0;
     struct mc_state *mcs = &current->mc_state;
@@ -1759,7 +1760,10 @@ int hypercall_xlat_continuation(unsigned int *id, unsigned int mask, ...)
     unsigned long nval = 0;
     va_list args;
 
-    BUG_ON(id && *id > 5);
+    ASSERT(nr <= ARRAY_SIZE(mcs->call.args));
+    ASSERT(!(mask >> nr));
+
+    BUG_ON(id && *id >= nr);
     BUG_ON(id && (mask & (1U << *id)));
 
     va_start(args, mask);
@@ -1772,7 +1776,7 @@ int hypercall_xlat_continuation(unsigned int *id, unsigned int mask, ...)
             return 0;
         }
 
-        for ( i = 0; i < 6; ++i, mask >>= 1 )
+        for ( i = 0; i < nr; ++i, mask >>= 1 )
         {
             if ( mask & 1 )
             {
@@ -1800,7 +1804,7 @@ int hypercall_xlat_continuation(unsigned int *id, unsigned int mask, ...)
     else
     {
         regs = guest_cpu_user_regs();
-        for ( i = 0; i < 6; ++i, mask >>= 1 )
+        for ( i = 0; i < nr; ++i, mask >>= 1 )
         {
             unsigned long *reg;
 

diff --git a/xen/arch/x86/x86_64/compat/mm.c b/xen/arch/x86/x86_64/compat/mm.c
index 54f25b75f07e3018d0d8981ca03e7a2f5d427c31..dce3f1fd02f0e9ef642fdf7748ae43d0278702b1 100644 (file)
--- a/xen/arch/x86/x86_64/compat/mm.c
+++ b/xen/arch/x86/x86_64/compat/mm.c
@@ -118,7 +118,7 @@ int compat_arch_memory_op(unsigned long cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
             break;
 
         if ( rc == __HYPERVISOR_memory_op )
-            hypercall_xlat_continuation(NULL, 0x2, nat, arg);
+            hypercall_xlat_continuation(NULL, 2, 0x2, nat, arg);
 
         XLAT_pod_target(&cmp, nat);
 
@@ -354,7 +354,7 @@ int compat_mmuext_op(XEN_GUEST_HANDLE_PARAM(mmuext_op_compat_t) cmp_uops,
                 left = 1;
                 if ( arg1 != MMU_UPDATE_PREEMPTED )
                 {
-                    BUG_ON(!hypercall_xlat_continuation(&left, 0x01, nat_ops,
+                    BUG_ON(!hypercall_xlat_continuation(&left, 4, 0x01, nat_ops,
                                                         cmp_uops));
                     if ( !test_bit(_MCSF_in_multicall, &mcs->flags) )
                         regs->_ecx += count - i;
@@ -362,7 +362,7 @@ int compat_mmuext_op(XEN_GUEST_HANDLE_PARAM(mmuext_op_compat_t) cmp_uops,
                         mcs->compat_call.args[1] += count - i;
                 }
                 else
-                    BUG_ON(hypercall_xlat_continuation(&left, 0));
+                    BUG_ON(hypercall_xlat_continuation(&left, 4, 0));
                 BUG_ON(left != arg1);
             }
             else

diff --git a/xen/common/compat/memory.c b/xen/common/compat/memory.c
index 43d02bcca6280e7121ecdde6f62376491052bb6d..06c90beebe346f9fb31a2c2f50409e1025497d75 100644 (file)
--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -282,7 +282,7 @@ int compat_memory_op(unsigned int cmd, XEN_GUEST_HANDLE_PARAM(void) compat)
             break;
 
         cmd = 0;
-        if ( hypercall_xlat_continuation(&cmd, 0x02, nat.hnd, compat) )
+        if ( hypercall_xlat_continuation(&cmd, 2, 0x02, nat.hnd, compat) )
         {
             BUG_ON(rc != __HYPERVISOR_memory_op);
             BUG_ON((cmd & MEMOP_CMD_MASK) != op);

diff --git a/xen/include/xen/compat.h b/xen/include/xen/compat.h
index d58aede8a97c6d7ecfcead80bf208212f40a103f..e5c23e2f9d2eb45bd0a3987e4c69526604cf6181 100644 (file)
--- a/xen/include/xen/compat.h
+++ b/xen/include/xen/compat.h
@@ -195,6 +195,8 @@ static inline int name(k xen_ ## n *x, k compat_ ## n *c) \
  * This option is useful for extracting the "op" argument or similar from the
  * hypercall to enable further xlat processing.
  *
+ * nr: Total number of arguments the hypercall has.
+ *
  * mask: Specifies which of the hypercall arguments require compat translation.
  * bit 0 indicates that the 0'th argument requires translation, bit 1 indicates
  * that the first argument requires translation and so on. Native and compat
@@ -214,7 +216,8 @@ static inline int name(k xen_ ## n *x, k compat_ ## n *c) \
  *
  * Return: Number of arguments which were actually translated.
  */
-int hypercall_xlat_continuation(unsigned int *id, unsigned int mask, ...);
+int hypercall_xlat_continuation(unsigned int *id, unsigned int nr,
+                                unsigned int mask, ...);
 
 /* In-place translation functons: */
 struct start_info;