[PATCH RFT] arm64: kasan: Make KASAN work with 16K pages + 48 bit VA

[Andrey Ryabinin]

Currently kasan assumes that shadow memory covers one or more entire PGDs.
That's not true for 16K pages + 48bit VA space, where PGDIR_SIZE is bigger
than the whole shadow memory.

This patch tries to fix that case.
clear_page_tables() is a new replacement of clear_pgs(). Instead of always
clearing pgds it clears top level page table entries that entirely belongs
to shadow memory.
In addition to 'tmp_pg_dir' we now have 'tmp_pud' which is used to store
puds that now might be cleared by clear_page_tables.

Reported-by: Suzuki K. Poulose <Suzuki.Poulose@xxxxxxx>
Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
---

 *** THIS is not tested with 16k pages ***

 arch/arm64/mm/kasan_init.c | 87 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 76 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index cf038c7..ea9f92a 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -22,6 +22,7 @@
 #include <asm/tlbflush.h>
 
 static pgd_t tmp_pg_dir[PTRS_PER_PGD] __initdata __aligned(PGD_SIZE);
+static pud_t tmp_pud[PAGE_SIZE/sizeof(pud_t)] __initdata __aligned(PAGE_SIZE);
 
 static void __init kasan_early_pte_populate(pmd_t *pmd, unsigned long addr,
 					unsigned long end)
@@ -92,20 +93,84 @@ asmlinkage void __init kasan_early_init(void)
 {
 	BUILD_BUG_ON(KASAN_SHADOW_OFFSET != KASAN_SHADOW_END - (1UL << 61));
 	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_START, PGDIR_SIZE));
-	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PGDIR_SIZE));
+	BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, PUD_SIZE));
 	kasan_map_early_shadow();
 }
 
-static void __init clear_pgds(unsigned long start,
-			unsigned long end)
+static void __init clear_pmds(pud_t *pud, unsigned long addr, unsigned long end)
 {
+	pmd_t *pmd;
+	unsigned long next;
+
+	pmd = pmd_offset(pud, addr);
+
+	do {
+		next = pmd_addr_end(addr, end);
+		if (IS_ALIGNED(addr, PMD_SIZE) && end - addr >= PMD_SIZE)
+			pmd_clear(pmd);
+
+	} while (pmd++, addr = next, addr != end);
+}
+
+static void __init clear_puds(pgd_t *pgd, unsigned long addr, unsigned long end)
+{
+	pud_t *pud;
+	unsigned long next;
+
+	pud = pud_offset(pgd, addr);
+
+	do {
+		next = pud_addr_end(addr, end);
+		if (IS_ALIGNED(addr, PUD_SIZE) && end - addr >= PUD_SIZE)
+			pud_clear(pud);
+
+		if (!pud_none(*pud))
+			clear_pmds(pud, addr, next);
+	} while (pud++, addr = next, addr != end);
+}
+
+static void __init clear_page_tables(unsigned long addr, unsigned long end)
+{
+	pgd_t *pgd;
+	unsigned long next;
+
+	pgd = pgd_offset_k(addr);
+
+	do {
+		next = pgd_addr_end(addr, end);
+		if (IS_ALIGNED(addr, PGDIR_SIZE) && end - addr >= PGDIR_SIZE)
+			pgd_clear(pgd);
+
+		if (!pgd_none(*pgd))
+			clear_puds(pgd, addr, next);
+	} while (pgd++, addr = next, addr != end);
+}
+
+static void copy_pagetables(void)
+{
+	pgd_t *pgd = tmp_pg_dir + pgd_index(KASAN_SHADOW_START);
+
+	memcpy(tmp_pg_dir, swapper_pg_dir, sizeof(tmp_pg_dir));
+
 	/*
-	 * Remove references to kasan page tables from
-	 * swapper_pg_dir. pgd_clear() can't be used
-	 * here because it's nop on 2,3-level pagetable setups
+	 * If kasan shadow shares PGD with other mappings,
+	 * clear_page_tables() will clear puds instead of pgd,
+	 * so we need temporary pud table to keep early shadow mapped.
 	 */
-	for (; start < end; start += PGDIR_SIZE)
-		set_pgd(pgd_offset_k(start), __pgd(0));
+	if (PGDIR_SIZE > KASAN_SHADOW_END - KASAN_SHADOW_START) {
+		pud_t *pud;
+		pmd_t *pmd;
+		pte_t *pte;
+
+		memcpy(tmp_pud, pgd_page_vaddr(*pgd), sizeof(tmp_pud));
+
+		pgd_populate(&init_mm, pgd, tmp_pud);
+		pud = pud_offset(pgd, KASAN_SHADOW_START);
+		pmd = pmd_offset(pud, KASAN_SHADOW_START);
+		pud_populate(&init_mm, pud, pmd);
+		pte = pte_offset_kernel(pmd, KASAN_SHADOW_START);
+		pmd_populate_kernel(&init_mm, pmd, pte);
+	}
 }
 
 static void __init cpu_set_ttbr1(unsigned long ttbr1)
@@ -123,16 +188,16 @@ void __init kasan_init(void)
 
 	/*
 	 * We are going to perform proper setup of shadow memory.
-	 * At first we should unmap early shadow (clear_pgds() call bellow).
+	 * At first we should unmap early shadow (clear_page_tables()).
 	 * However, instrumented code couldn't execute without shadow memory.
 	 * tmp_pg_dir used to keep early shadow mapped until full shadow
 	 * setup will be finished.
 	 */
-	memcpy(tmp_pg_dir, swapper_pg_dir, sizeof(tmp_pg_dir));
+	copy_pagetables();
 	cpu_set_ttbr1(__pa(tmp_pg_dir));
 	flush_tlb_all();
 
-	clear_pgds(KASAN_SHADOW_START, KASAN_SHADOW_END);
+	clear_page_tables(KASAN_SHADOW_START, KASAN_SHADOW_END);
 
 	kasan_populate_zero_shadow((void *)KASAN_SHADOW_START,
 			kasan_mem_to_shadow((void *)MODULES_VADDR));
-- 
2.4.10

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