On Wed,  7 Mar 2018 11:32:26 -0700 Toshi Kani [off-list ref] wrote:

On architectures with CONFIG_HAVE_ARCH_HUGE_VMAP set, ioremap()
may create pud/pmd mappings.  Kernel panic was observed on arm64
systems with Cortex-A75 in the following steps as described by
Hanjun Guo.

1. ioremap a 4K size, valid page table will build,
2. iounmap it, pte0 will set to 0;
3. ioremap the same address with 2M size, pgd/pmd is unchanged,
   then set the a new value for pmd;
4. pte0 is leaked;
5. CPU may meet exception because the old pmd is still in TLB,
   which will lead to kernel panic.

This panic is not reproducible on x86.  INVLPG, called from iounmap,
purges all levels of entries associated with purged address on x86.
x86 still has memory leak.

Add two interfaces, pud_free_pmd_page() and pmd_free_pte_page(),
which clear a given pud/pmd entry and free up a page for the lower
level entries.

This patch implements their stub functions on x86 and arm64, which
work as workaround.

Also, as this fixes an arm64 kernel panic, should we be using cc:stable?