x86/mm: Don't perform flush after failing to update a guests L1e

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 <mattd@bugfuzz.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 6c8d50288722672ecc8e19b0741a31b521d01706
master date: 2018-11-20 14:58:41 +0100

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 66530c8a9ba14d02765d28e175651bdb8d972e7d..642dde49119eb5187a5cefa7ab6d618af1b6f7e7 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -4840,6 +4840,14 @@ static int __do_update_va_mapping(
     if ( pl1e )
         guest_unmap_l1e(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: