[PATCH SpectreV1+L1TF v7 8/9] x86/hvm: add nospec to hvmop param

[Norbert Manthey <nmanthey@amazon.de>]

The params array in hvm can be accessed with get and set functions.
As the index is guest controlled, make sure no out-of-bound accesses
can be performed.

As we cannot influence how future compilers might modify the
instructions that enforce the bounds, we furthermore block speculation,
so that the update is visible in the architectural state.

This is part of the speculative hardening effort.

Signed-off-by: Norbert Manthey <nmanthey@amazon.de>

---

Notes:
  v7: add speculative hardening to commit message
      add lfence to a.index updates (other parts of that commit have
          been committed already)

 xen/arch/x86/hvm/hvm.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4109,6 +4109,13 @@ static int hvmop_set_param(
     if ( a.index >= HVM_NR_PARAMS )
         return -EINVAL;
 
+    /*
+     * Make sure the guest controlled value a.index is bounded even during
+     * speculative execution.
+     */
+    a.index = array_index_nospec(a.index, HVM_NR_PARAMS);
+    block_speculation();
+
     d = rcu_lock_domain_by_any_id(a.domid);
     if ( d == NULL )
         return -ESRCH;
@@ -4375,6 +4382,13 @@ static int hvmop_get_param(
     if ( a.index >= HVM_NR_PARAMS )
         return -EINVAL;
 
+    /*
+     * Make sure the guest controlled value a.index is bounded even during
+     * speculative execution.
+     */
+    a.index = array_index_nospec(a.index, HVM_NR_PARAMS);
+    block_speculation();
+
     d = rcu_lock_domain_by_any_id(a.domid);
     if ( d == NULL )
         return -ESRCH;
-- 
2.7.4




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