Patch "mm/vmalloc: add interfaces to free unmapped page table" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

mm/vmalloc: add interfaces to free unmapped page table

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-vmalloc-add-interfaces-to-free-unmapped-page-table.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Toshi Kani <[email protected]>
Date: Thu, 22 Mar 2018 16:17:20 -0700
Subject: mm/vmalloc: add interfaces to free unmapped page table

From: Toshi Kani <[email protected]>

commit b6bdb7517c3d3f41f20e5c2948d6bc3f8897394e upstream.

On architectures with CONFIG_HAVE_ARCH_HUGE_VMAP set, ioremap() may create pud/pmd mappings. A 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.

---
 arch/arm64/mm/mmu.c           |   10 ++++++++++
 arch/x86/mm/pgtable.c         |   24 ++++++++++++++++++++++++
 include/asm-generic/pgtable.h |   10 ++++++++++
 lib/ioremap.c                 |    6 ++++--
 4 files changed, 48 insertions(+), 2 deletions(-)

@@ -937,3 +937,13 @@ int pmd_clear_huge(pmd_t *pmd)
 	pmd_clear(pmd);
 	return 1;
 }
+
+int pud_free_pmd_page(pud_t *pud)
+{
+	return pud_none(*pud);
+}
+
+int pmd_free_pte_page(pmd_t *pmd)
+{
+	return pmd_none(*pmd);
+}

@@ -702,4 +702,28 @@ int pmd_clear_huge(pmd_t *pmd)
 
 	return 0;
 }
+
+/**
+ * pud_free_pmd_page - Clear pud entry and free pmd page.
+ * @pud: Pointer to a PUD.
+ *
+ * Context: The pud range has been unmaped and TLB purged.
+ * Return: 1 if clearing the entry succeeded. 0 otherwise.
+ */
+int pud_free_pmd_page(pud_t *pud)
+{
+	return pud_none(*pud);
+}
+
+/**
+ * pmd_free_pte_page - Clear pmd entry and free pte page.
+ * @pmd: Pointer to a PMD.
+ *
+ * Context: The pmd range has been unmaped and TLB purged.
+ * Return: 1 if clearing the entry succeeded. 0 otherwise.
+ */
+int pmd_free_pte_page(pmd_t *pmd)
+{
+	return pmd_none(*pmd);
+}
 #endif	/* CONFIG_HAVE_ARCH_HUGE_VMAP */

@@ -976,6 +976,8 @@ int pud_set_huge(pud_t *pud, phys_addr_t
 int pmd_set_huge(pmd_t *pmd, phys_addr_t addr, pgprot_t prot);
 int pud_clear_huge(pud_t *pud);
 int pmd_clear_huge(pmd_t *pmd);
+int pud_free_pmd_page(pud_t *pud);
+int pmd_free_pte_page(pmd_t *pmd);
 #else	/* !CONFIG_HAVE_ARCH_HUGE_VMAP */
 static inline int p4d_set_huge(p4d_t *p4d, phys_addr_t addr, pgprot_t prot)
 {

@@ -1001,6 +1003,14 @@ static inline int pmd_clear_huge(pmd_t *
 {
 	return 0;
 }
+static inline int pud_free_pmd_page(pud_t *pud)
+{
+	return 0;
+}
+static inline int pmd_free_pte_page(pmd_t *pmd)
+{
+	return 0;
+}
 #endif	/* CONFIG_HAVE_ARCH_HUGE_VMAP */
 
 #ifndef __HAVE_ARCH_FLUSH_PMD_TLB_RANGE

@@ -91,7 +91,8 @@ static inline int ioremap_pmd_range(pud_
 
 		if (ioremap_pmd_enabled() &&
 		    ((next - addr) == PMD_SIZE) &&
-		    IS_ALIGNED(phys_addr + addr, PMD_SIZE)) {
+		    IS_ALIGNED(phys_addr + addr, PMD_SIZE) &&
+		    pmd_free_pte_page(pmd)) {
 			if (pmd_set_huge(pmd, phys_addr + addr, prot))
 				continue;
 		}

@@ -117,7 +118,8 @@ static inline int ioremap_pud_range(p4d_
 
 		if (ioremap_pud_enabled() &&
 		    ((next - addr) == PUD_SIZE) &&
-		    IS_ALIGNED(phys_addr + addr, PUD_SIZE)) {
+		    IS_ALIGNED(phys_addr + addr, PUD_SIZE) &&
+		    pud_free_pmd_page(pud)) {
 			if (pud_set_huge(pud, phys_addr + addr, prot))
 				continue;
 		}

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/mm-vmalloc-add-interfaces-to-free-unmapped-page-table.patch queue-4.14/x86-mm-implement-free-pmd-pte-page-interfaces.patch