arm64/mm: Inhibit huge-vmap with ptdump

This change “arm64/mm: Inhibit huge-vmap with ptdump” (commit 7ba36ec) in Linux kernel is authored by Mark Rutland <mark.rutland [at] arm.com> on Tue May 14 14:30:06 2019 +0530.

Description of "arm64/mm: Inhibit huge-vmap with ptdump"

The change “arm64/mm: Inhibit huge-vmap with ptdump” introduces changes as follows.

arm64/mm: Inhibit huge-vmap with ptdump

The arm64 ptdump code can race with concurrent modification of the
kernel page tables. At the time this was added, this was sound as:

* Modifications to leaf entries could result in stale information being
  logged, but would not result in a functional problem.

* Boot time modifications to non-leaf entries (e.g. freeing of initmem)
  were performed when the ptdump code cannot be invoked.

* At runtime, modifications to non-leaf entries only occurred in the
  vmalloc region, and these were strictly additive, as intermediate
  entries were never freed.

However, since commit:

  commit 324420bf91f6 ("arm64: add support for ioremap() block mappings")

... it has been possible to create huge mappings in the vmalloc area at
runtime, and as part of this existing intermediate levels of table my be
removed and freed.

It's possible for the ptdump code to race with this, and continue to
walk tables which have been freed (and potentially poisoned or
reallocated). As a result of this, the ptdump code may dereference bogus
addresses, which could be fatal.

Since huge-vmap is a TLB and memory optimization, we can disable it when
the runtime ptdump code is in use to avoid this problem.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 324420bf91f60582 ("arm64: add support for ioremap() block mappings")
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Linux kernel releases containing commit 7ba36ec

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "arm64/mm: Inhibit huge-vmap with ptdump"

There are 11 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/arm64/mm/mmu.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
 
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index ef82312860ac..37a902cad918 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -955,13 +955,18 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
 
 int __init arch_ioremap_pud_supported(void)
 {
-	/* only 4k granule supports level 1 block mappings */
-	return IS_ENABLED(CONFIG_ARM64_4K_PAGES);
+	/*
+	 * Only 4k granule supports level 1 block mappings.
+	 * SW table walks can't handle removal of intermediate entries.
+	 */
+	return IS_ENABLED(CONFIG_ARM64_4K_PAGES) &&
+	       !IS_ENABLED(CONFIG_ARM64_PTDUMP_DEBUGFS);
 }
 
 int __init arch_ioremap_pmd_supported(void)
 {
-	return 1;
+	/* See arch_ioremap_pud_supported() */
+	return !IS_ENABLED(CONFIG_ARM64_PTDUMP_DEBUGFS);
 }
 
 int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)

The commit for this change in Linux stable tree is 7ba36ec (patch).

Last modified: 2020/02/08