[PATCH v2 1/2] mm/vmalloc: Add interfaces to free unmapped page table

[Andrew Morton]

On Wed, 14 Mar 2018 12:01:54 -0600 Toshi Kani <toshi.kani at hpe.com> 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.
> 
> The patch changes the ioremap path to free unmapped page table(s) since
> doing so in the unmap path has the following issues:
> 
>  - The iounmap() path is shared with vunmap().  Since vmap() only
>    supports pte mappings, making vunmap() to free a pte page is an
>    overhead for regular vmap users as they do not need a pte page
>    freed up.
>  - Checking if all entries in a pte page are cleared in the unmap path
>    is racy, and serializing this check is expensive.
>  - The unmap path calls free_vmap_area_noflush() to do lazy TLB purges.
>    Clearing a pud/pmd entry before the lazy TLB purges needs extra TLB
>    purge.
> 
> 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.
> 

whoops.

--- a/include/asm-generic/pgtable.h~mm-vmalloc-add-interfaces-to-free-unmapped-page-table-fix
+++ a/include/asm-generic/pgtable.h
@@ -1014,7 +1014,7 @@ static inline int pud_free_pmd_page(pud_
 {
 	return 0;
 }
-static inline int pmd_free_pte_page(pud_t *pmd)
+static inline int pmd_free_pte_page(pmd_t *pmd)
 {
 	return 0;
 }
_