arm64: mm: introduce present, faulting entries for PAGE_NONE

This is mostly a port of dbf62d5 ("ARM: mm: introduce L_PTE_VALID
for page table entries") and 26ffd0d ("ARM: mm: introduce present,
faulting entries for PAGE_NONE") from ARM, which makes use of present,
faulting page table entries for page table entries mapped as PROT_NONE.

The main difference with this implementation is that we can make use of
the two pte type bits in order to avoid allocating a software bit for
identifying PROT_NONE pages, instead reserving the 10b suffix for these
types of mappings.

This is required to prevent users from accessing such pages via syscalls
such as read/write over a pipe.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -24,7 +24,8 @@
 /*
  * Software defined PTE bits definition.
  */
-#define PTE_VALID		(_AT(pteval_t, 1) << 0)	/* pte_present() check */
+#define PTE_VALID		(_AT(pteval_t, 1) << 0)
+#define PTE_PROT_NONE		(_AT(pteval_t, 1) << 1)	/* only when !PTE_VALID */
 #define PTE_FILE		(_AT(pteval_t, 1) << 2)	/* only when !pte_present() */
 #define PTE_DIRTY		(_AT(pteval_t, 1) << 55)
 #define PTE_SPECIAL		(_AT(pteval_t, 1) << 56)
@@ -60,9 +61,12 @@ extern void __pgd_error(const char *file, int line, unsigned long val);
 
 extern pgprot_t pgprot_default;
 
-#define _MOD_PROT(p, b)	__pgprot(pgprot_val(p) | (b))
+#define __pgprot_modify(prot,mask,bits) \
+	__pgprot((pgprot_val(prot) & ~(mask)) | (bits))
+
+#define _MOD_PROT(p, b)		__pgprot_modify(p, 0, b)
 
-#define PAGE_NONE		_MOD_PROT(pgprot_default, PTE_NG | PTE_PXN | PTE_UXN | PTE_RDONLY)
+#define PAGE_NONE		__pgprot_modify(pgprot_default, PTE_TYPE_MASK, PTE_PROT_NONE)
 #define PAGE_SHARED		_MOD_PROT(pgprot_default, PTE_USER | PTE_NG | PTE_PXN | PTE_UXN)
 #define PAGE_SHARED_EXEC	_MOD_PROT(pgprot_default, PTE_USER | PTE_NG | PTE_PXN)
 #define PAGE_COPY		_MOD_PROT(pgprot_default, PTE_USER | PTE_NG | PTE_PXN | PTE_UXN | PTE_RDONLY)
@@ -72,7 +76,7 @@ extern pgprot_t pgprot_default;
 #define PAGE_KERNEL		_MOD_PROT(pgprot_default, PTE_PXN | PTE_UXN | PTE_DIRTY)
 #define PAGE_KERNEL_EXEC	_MOD_PROT(pgprot_default, PTE_UXN | PTE_DIRTY)
 
-#define __PAGE_NONE		__pgprot(_PAGE_DEFAULT | PTE_NG | PTE_PXN | PTE_UXN | PTE_RDONLY)
+#define __PAGE_NONE		__pgprot(((_PAGE_DEFAULT) & ~PTE_TYPE_MASK) | PTE_PROT_NONE)
 #define __PAGE_SHARED		__pgprot(_PAGE_DEFAULT | PTE_USER | PTE_NG | PTE_PXN | PTE_UXN)
 #define __PAGE_SHARED_EXEC	__pgprot(_PAGE_DEFAULT | PTE_USER | PTE_NG | PTE_PXN)
 #define __PAGE_COPY		__pgprot(_PAGE_DEFAULT | PTE_USER | PTE_NG | PTE_PXN | PTE_UXN | PTE_RDONLY)
@@ -125,14 +129,14 @@ extern struct page *empty_zero_page;
 /*
  * The following only work if pte_present(). Undefined behaviour otherwise.
  */
-#define pte_present(pte)	(pte_val(pte) & PTE_VALID)
+#define pte_present(pte)	(pte_val(pte) & (PTE_VALID | PTE_PROT_NONE))
 #define pte_dirty(pte)		(pte_val(pte) & PTE_DIRTY)
 #define pte_young(pte)		(pte_val(pte) & PTE_AF)
 #define pte_special(pte)	(pte_val(pte) & PTE_SPECIAL)
 #define pte_write(pte)		(!(pte_val(pte) & PTE_RDONLY))
 #define pte_exec(pte)		(!(pte_val(pte) & PTE_UXN))
 
-#define pte_present_user(pte) \
+#define pte_valid_user(pte) \
 	((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER))
 
 #define PTE_BIT_FUNC(fn,op) \
@@ -156,7 +160,7 @@ extern void __sync_icache_dcache(pte_t pteval, unsigned long addr);
 static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 			      pte_t *ptep, pte_t pte)
 {
-	if (pte_present_user(pte)) {
+	if (pte_valid_user(pte)) {
 		if (pte_exec(pte))
 			__sync_icache_dcache(pte, addr);
 		if (!pte_dirty(pte))
@@ -172,9 +176,6 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 #define pte_huge(pte)		((pte_val(pte) & PTE_TYPE_MASK) == PTE_TYPE_HUGEPAGE)
 #define pte_mkhuge(pte)		(__pte((pte_val(pte) & ~PTE_TYPE_MASK) | PTE_TYPE_HUGEPAGE))
 
-#define __pgprot_modify(prot,mask,bits)		\
-	__pgprot((pgprot_val(prot) & ~(mask)) | (bits))
-
 #define __HAVE_ARCH_PTE_SPECIAL
 
 /*
@@ -266,7 +267,8 @@ static inline pmd_t *pmd_offset(pud_t *pud, unsigned long addr)
 
 static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 {
-	const pteval_t mask = PTE_USER | PTE_PXN | PTE_UXN | PTE_RDONLY;
+	const pteval_t mask = PTE_USER | PTE_PXN | PTE_UXN | PTE_RDONLY |
+			      PTE_PROT_NONE | PTE_VALID;
 	pte_val(pte) = (pte_val(pte) & ~mask) | (pgprot_val(newprot) & mask);
 	return pte;
 }