net: dsa: sja1105: fix kasan out-of-bounds warning in sja1105_table_delete_entry()

There are actually 2 problems:
- deleting the last element doesn't require the memmove of elements
  [i + 1, end) over it. Actually, element i+1 is out of bounds.
- The memmove itself should move size - i - 1 elements, because the last
  element is out of bounds.

The out-of-bounds element still remains out of bounds after being
accessed, so the problem is only that we touch it, not that it becomes
in active use. But I suppose it can lead to issues if the out-of-bounds
element is part of an unmapped page.

Issue: ALBSP-454
Fixes: 6666ceb ("net: dsa: sja1105: Add support for VLAN operations")
Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: vladimiroltean

drivers/net/dsa/sja1105/sja1105_static_config.c

@@ -1921,8 +1921,10 @@ int sja1105_table_delete_entry(struct sja1105_table *table, int i)
 	if (i > table->entry_count)
 		return -ERANGE;
 
-	memmove(entries + i * entry_size, entries + (i + 1) * entry_size,
-		(table->entry_count - i) * entry_size);
+	if (i + 1 < table->entry_count) {
+		memmove(entries + i * entry_size, entries + (i + 1) * entry_size,
+			(table->entry_count - i - 1) * entry_size);
+	}
 
 	table->entry_count--;