[RFC PATCH 2/2] arm64: Notify on pte permission upgrades

[Alistair Popple]

ARM64 requires TLB invalidates when upgrading pte permission from
read-only to read-write. However mmu_notifiers assume upgrades do not
need notifications and none are sent. This causes problems when a
secondary TLB such as implemented by an ARM SMMU doesn't support
broadcast TLB maintenance (BTM) and caches a read-only PTE.

As no notification is sent and the SMMU does not snoop TLB invalidates
it will continue to return read-only entries to a device even though
the CPU page table contains a writable entry. This leads to a
continually faulting device and no way of handling the fault.

The ARM SMMU driver already registers for mmu notifier events to keep
any secondary TLB synchronised. Therefore sending a notifier on
permission upgrade fixes the problem.

Rather than adding notifier calls to generic architecture independent
code where it may cause performance regressions on architectures that
don't require it add it to the architecture specific
ptep_set_access_flags() where the CPU TLB is invalidated.

Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
---
 arch/arm64/mm/fault.c       |  7 ++++++-
 arch/arm64/mm/hugetlbpage.c |  9 +++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index c601007..c28f257 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -25,6 +25,7 @@
 #include <linux/perf_event.h>
 #include <linux/preempt.h>
 #include <linux/hugetlb.h>
+#include <linux/mmu_notifier.h>
 
 #include <asm/acpi.h>
 #include <asm/bug.h>
@@ -239,8 +240,12 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 	} while (pteval != old_pteval);
 
 	/* Invalidate a stale read-only entry */
-	if (dirty)
+	if (dirty) {
 		flush_tlb_page(vma, address);
+		mmu_notifier_invalidate_secondary_tlbs(vma->vm_mm,
+					address & PAGE_MASK,
+					(address & PAGE_MASK) + PAGE_SIZE);
+	}
 	return 1;
 }
 
diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 21716c9..b689406 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -14,6 +14,7 @@
 #include <linux/pagemap.h>
 #include <linux/err.h>
 #include <linux/sysctl.h>
+#include <linux/mmu_notifier.h>
 #include <asm/mman.h>
 #include <asm/tlb.h>
 #include <asm/tlbflush.h>
@@ -480,6 +481,14 @@ int huge_ptep_set_access_flags(struct vm_area_struct *vma,
 
 	orig_pte = get_clear_contig_flush(mm, addr, ptep, pgsize, ncontig);
 
+	/*
+	 * Make sure any cached read-only entries are removed from
+	 * secondary TLBs.
+	 */
+	if (dirty)
+		mmu_notifier_invalidate_secondary_tlbs(mm, addr,
+						addr + (pgsize + ncontig));
+
 	/* Make sure we don't lose the dirty or young state */
 	if (pte_dirty(orig_pte))
 		pte = pte_mkdirty(pte);
-- 
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