Subject: x86/mm: Don't perform flush after failing to update a guests L1e
From: Andrew Cooper <[email-masked]>

If the L1e update hasn't occured, the flush cannot do anything useful.  This
skips the potentially expensive vcpumask_to_pcpumask() conversion, and
broadcast TLB shootdown.

More importantly however, we might be in the error path due to a bad va
parameter from the guest, and this should not propagate into the TLB flushing
logic.  The INVPCID instruction for example raises #GP for a non-canonical
address.

This is XSA-279.

Reported-by: Matthew Daley <[email-masked]>
Signed-off-by: Andrew Cooper <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 703f330..75663c6 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4155,6 +4155,14 @@ static int __do_update_va_mapping(
     if ( pl1e )
         unmap_domain_page(pl1e);
 
+    /*
+     * Any error at this point means that we haven't change the L1e.  Skip the
+     * flush, as it won't do anything useful.  Furthermore, va is guest
+     * controlled and not necesserily audited by this point.
+     */
+    if ( rc )
+        return rc;
+
     switch ( flags & UVMF_FLUSHTYPE_MASK )
     {
     case UVMF_TLB_FLUSH: