  We provide an overview of recent developments and future directions in the
RowHammer vulnerability that plagues modern DRAM (Dynamic Random Memory Access)
chips, which are used in almost all computing systems as main memory.
  RowHammer is the phenomenon in which repeatedly accessing a row in a real
DRAM chip causes bitflips (i.e., data corruption) in physically nearby rows.
This phenomenon leads to a serious and widespread system security
vulnerability, as many works since the original RowHammer paper in 2014 have
shown. Recent analysis of the RowHammer phenomenon reveals that the problem is
getting much worse as DRAM technology scaling continues: newer DRAM chips are
fundamentally more vulnerable to RowHammer at the device and circuit levels.
Deeper analysis of RowHammer shows that there are many dimensions to the
problem as the vulnerability is sensitive to many variables, including
environmental conditions (temperature \& voltage), process variation, stored
data patterns, as well as memory access patterns and memory control policies.
As such, it has proven difficult to devise fully-secure and very efficient
(i.e., low-overhead in performance, energy, area) protection mechanisms against
RowHammer and attempts made by DRAM manufacturers have been shown to lack
security guarantees.
  After reviewing various recent developments in exploiting, understanding, and
mitigating RowHammer, we discuss future directions that we believe are critical
for solving the RowHammer problem. We argue for two major directions to amplify
research and development efforts in: 1) building a much deeper understanding of
the problem and its many dimensions, in both cutting-edge DRAM chips and
computing systems deployed in the field, and 2) the design and development of
extremely efficient and fully-secure solutions via system-memory cooperation.
