[PATCH 2/3] xen/livepatch: do Xen build-id check earlier

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The check against the expected Xen build ID should be done ahead of attempting
to apply the alternatives contained in the livepatch.

If the CPUID in the alternatives patching data is out of the scope of the
running Xen featureset the BUG() in _apply_alternatives() will trigger thus
bringing the system down.  Note the layout of struct alt_instr could also
change between versions.  It's also possible for struct exception_table_entry
to have changed format, hence possibly leading to other errors.

Move the Xen build ID check to be done ahead of any processing of the livepatch
payload sections.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/common/livepatch.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index cea47ffe4c84..3e4fce036a1c 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -767,6 +767,11 @@ static int prepare_payload(struct payload *payload,
     if ( rc )
         return rc;
 
+    /* Perform the Xen build-id check ahead of doing any more processing. */
+    rc = xen_build_id_dep(payload);
+    if ( rc )
+        return rc;
+
     /* Setup the virtual region with proper data. */
     region = &payload->region;
 
@@ -1099,10 +1104,6 @@ static int load_payload_data(struct payload *payload, 
void *raw, size_t len)
     if ( rc )
         goto out;
 
-    rc = xen_build_id_dep(payload);
-    if ( rc )
-        goto out;
-
     rc = build_symbol_table(payload, &elf);
     if ( rc )
         goto out;
-- 
2.46.0