arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables

In many cases, page tables can be accessed concurrently by either another
CPU (due to things like fast gup) or by the hardware page table walker
itself, which may set access/dirty bits. In such cases, it is important
to use READ_ONCE/WRITE_ONCE when accessing page table entries so that
entries cannot be torn, merged or subject to apparent loss of coherence
due to compiler transformations.

Whilst there are some scenarios where this cannot happen (e.g. pinned
kernel mappings for the linear region), the overhead of using READ_ONCE
/WRITE_ONCE everywhere is minimal and makes the code an awful lot easier
to reason about. This patch consistently uses these macros in the arch
code, as well as explicitly namespacing pointers to page table entries
from the entries themselves by using adopting a 'p' suffix for the former
(as is sometimes used elsewhere in the kernel source).

Tested-by: Yury Norov <ynorov@caviumnetworks.com>
Tested-by: Richard Ruigrok <rruigrok@codeaurora.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 30 insertions, 24 deletions

diff --git a/arch/arm64/mm/dump.c b/arch/arm64/mm/dump.c
index 7b60d62ac593..65dfc8571bf8 100644
--- a/arch/arm64/mm/dump.c
+++ b/arch/arm64/mm/dump.c
@@ -286,48 +286,52 @@ static void note_page(struct pg_state *st, unsigned long addr, unsigned level,
 
 }
 
-static void walk_pte(struct pg_state *st, pmd_t *pmd, unsigned long start)
+static void walk_pte(struct pg_state *st, pmd_t *pmdp, unsigned long start)
 {
-	pte_t *pte = pte_offset_kernel(pmd, 0UL);
+	pte_t *ptep = pte_offset_kernel(pmdp, 0UL);
 	unsigned long addr;
 	unsigned i;
 
-	for (i = 0; i < PTRS_PER_PTE; i++, pte++) {
+	for (i = 0; i < PTRS_PER_PTE; i++, ptep++) {
 		addr = start + i * PAGE_SIZE;
-		note_page(st, addr, 4, pte_val(*pte));
+		note_page(st, addr, 4, READ_ONCE(pte_val(*ptep)));
 	}
 }
 
-static void walk_pmd(struct pg_state *st, pud_t *pud, unsigned long start)
+static void walk_pmd(struct pg_state *st, pud_t *pudp, unsigned long start)
 {
-	pmd_t *pmd = pmd_offset(pud, 0UL);
+	pmd_t *pmdp = pmd_offset(pudp, 0UL);
 	unsigned long addr;
 	unsigned i;
 
-	for (i = 0; i < PTRS_PER_PMD; i++, pmd++) {
+	for (i = 0; i < PTRS_PER_PMD; i++, pmdp++) {
+		pmd_t pmd = READ_ONCE(*pmdp);
+
 		addr = start + i * PMD_SIZE;
-		if (pmd_none(*pmd) || pmd_sect(*pmd)) {
-			note_page(st, addr, 3, pmd_val(*pmd));
+		if (pmd_none(pmd) || pmd_sect(pmd)) {
+			note_page(st, addr, 3, pmd_val(pmd));
 		} else {
-			BUG_ON(pmd_bad(*pmd));
-			walk_pte(st, pmd, addr);
+			BUG_ON(pmd_bad(pmd));
+			walk_pte(st, pmdp, addr);
 		}
 	}
 }
 
-static void walk_pud(struct pg_state *st, pgd_t *pgd, unsigned long start)
+static void walk_pud(struct pg_state *st, pgd_t *pgdp, unsigned long start)
 {
-	pud_t *pud = pud_offset(pgd, 0UL);
+	pud_t *pudp = pud_offset(pgdp, 0UL);
 	unsigned long addr;
 	unsigned i;
 
-	for (i = 0; i < PTRS_PER_PUD; i++, pud++) {
+	for (i = 0; i < PTRS_PER_PUD; i++, pudp++) {
+		pud_t pud = READ_ONCE(*pudp);
+
 		addr = start + i * PUD_SIZE;
-		if (pud_none(*pud) || pud_sect(*pud)) {
-			note_page(st, addr, 2, pud_val(*pud));
+		if (pud_none(pud) || pud_sect(pud)) {
+			note_page(st, addr, 2, pud_val(pud));
 		} else {
-			BUG_ON(pud_bad(*pud));
-			walk_pmd(st, pud, addr);
+			BUG_ON(pud_bad(pud));
+			walk_pmd(st, pudp, addr);
 		}
 	}
 }
@@ -335,17 +339,19 @@ static void walk_pud(struct pg_state *st, pgd_t *pgd, unsigned long start)
 static void walk_pgd(struct pg_state *st, struct mm_struct *mm,
 		     unsigned long start)
 {
-	pgd_t *pgd = pgd_offset(mm, 0UL);
+	pgd_t *pgdp = pgd_offset(mm, 0UL);
 	unsigned i;
 	unsigned long addr;
 
-	for (i = 0; i < PTRS_PER_PGD; i++, pgd++) {
+	for (i = 0; i < PTRS_PER_PGD; i++, pgdp++) {
+		pgd_t pgd = READ_ONCE(*pgdp);
+
 		addr = start + i * PGDIR_SIZE;
-		if (pgd_none(*pgd)) {
-			note_page(st, addr, 1, pgd_val(*pgd));
+		if (pgd_none(pgd)) {
+			note_page(st, addr, 1, pgd_val(pgd));
 		} else {
-			BUG_ON(pgd_bad(*pgd));
-			walk_pud(st, pgd, addr);
+			BUG_ON(pgd_bad(pgd));
+			walk_pud(st, pgdp, addr);
 		}
 	}
 }