[S390] noexec protection

This provides a noexec protection on s390 hardware. Our hardware does
not have any bits left in the pte for a hw noexec bit, so this is a
different approach using shadow page tables and a special addressing
mode that allows separate address spaces for code and data.

As a special feature of our "secondary-space" addressing mode, separate
page tables can be specified for the translation of data addresses
(storage operands) and instruction addresses. The shadow page table is
used for the instruction addresses and the standard page table for the
data addresses.
The shadow page table is linked to the standard page table by a pointer
in page->lru.next of the struct page corresponding to the page that
contains the standard page table (since page->private is not really
private with the pte_lock and the page table pages are not in the LRU
list).
Depending on the software bits of a pte, it is either inserted into
both page tables or just into the standard (data) page table. Pages of
a vma that does not have the VM_EXEC bit set get mapped only in the
data address space. Any try to execute code on such a page will cause a
page translation exception. The standard reaction to this is a SIGSEGV
with two exceptions: the two system call opcodes 0x0a77 (sys_sigreturn)
and 0x0aad (sys_rt_sigreturn) are allowed. They are stored by the
kernel to the signal stack frame. Unfortunately, the signal return
mechanism cannot be modified to use an SA_RESTORER because the
exception unwinding code depends on the system call opcode stored
behind the signal stack frame.

This feature requires that user space is executed in secondary-space
mode and the kernel in home-space mode, which means that the addressing
modes need to be switched and that the noexec protection only works
for user space.
After switching the addressing modes, we cannot use the mvcp/mvcs
instructions anymore to copy between kernel and user space. A new
mvcos instruction has been added to the z9 EC/BC hardware which allows
to copy between arbitrary address spaces, but on older hardware the
page tables need to be walked manually.

Signed-off-by: Gerald Schaefer <geraldsc@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 130 insertions, 16 deletions

diff --git a/include/asm-s390/pgtable.h b/include/asm-s390/pgtable.h
index 304ee77..13c1654 100644
--- a/include/asm-s390/pgtable.h
+++ b/include/asm-s390/pgtable.h
@@ -224,6 +224,8 @@ extern unsigned long vmalloc_end;
 #define _PAGE_TYPE_FILE		0x601	/* bit 0x002 is used for offset !! */
 #define _PAGE_TYPE_RO		0x200
 #define _PAGE_TYPE_RW		0x000
+#define _PAGE_TYPE_EX_RO	0x202
+#define _PAGE_TYPE_EX_RW	0x002
 
 /*
  * PTE type bits are rather complicated. handle_pte_fault uses pte_present,
@@ -244,11 +246,13 @@ extern unsigned long vmalloc_end;
  * _PAGE_TYPE_FILE	11?1   ->   11?1
  * _PAGE_TYPE_RO	0100   ->   1100
  * _PAGE_TYPE_RW	0000   ->   1000
+ * _PAGE_TYPE_EX_RO	0110   ->   1110
+ * _PAGE_TYPE_EX_RW	0010   ->   1010
  *
- * pte_none is true for bits combinations 1000, 1100
+ * pte_none is true for bits combinations 1000, 1010, 1100, 1110
  * pte_present is true for bits combinations 0000, 0010, 0100, 0110, 1001
  * pte_file is true for bits combinations 1101, 1111
- * swap pte is 1011 and 0001, 0011, 0101, 0111, 1010 and 1110 are invalid.
+ * swap pte is 1011 and 0001, 0011, 0101, 0111 are invalid.
  */
 
 #ifndef __s390x__
@@ -313,33 +317,100 @@ extern unsigned long vmalloc_end;
 #define PAGE_NONE	__pgprot(_PAGE_TYPE_NONE)
 #define PAGE_RO		__pgprot(_PAGE_TYPE_RO)
 #define PAGE_RW		__pgprot(_PAGE_TYPE_RW)
+#define PAGE_EX_RO	__pgprot(_PAGE_TYPE_EX_RO)
+#define PAGE_EX_RW	__pgprot(_PAGE_TYPE_EX_RW)
 
 #define PAGE_KERNEL	PAGE_RW
 #define PAGE_COPY	PAGE_RO
 
 /*
- * The S390 can't do page protection for execute, and considers that the
- * same are read. Also, write permissions imply read permissions. This is
- * the closest we can get..
+ * Dependent on the EXEC_PROTECT option s390 can do execute protection.
+ * Write permission always implies read permission. In theory with a
+ * primary/secondary page table execute only can be implemented but
+ * it would cost an additional bit in the pte to distinguish all the
+ * different pte types. To avoid that execute permission currently
+ * implies read permission as well.
  */
          /*xwr*/
 #define __P000	PAGE_NONE
 #define __P001	PAGE_RO
 #define __P010	PAGE_RO
 #define __P011	PAGE_RO
-#define __P100	PAGE_RO
-#define __P101	PAGE_RO
-#define __P110	PAGE_RO
-#define __P111	PAGE_RO
+#define __P100	PAGE_EX_RO
+#define __P101	PAGE_EX_RO
+#define __P110	PAGE_EX_RO
+#define __P111	PAGE_EX_RO
 
 #define __S000	PAGE_NONE
 #define __S001	PAGE_RO
 #define __S010	PAGE_RW
 #define __S011	PAGE_RW
-#define __S100	PAGE_RO
-#define __S101	PAGE_RO
-#define __S110	PAGE_RW
-#define __S111	PAGE_RW
+#define __S100	PAGE_EX_RO
+#define __S101	PAGE_EX_RO
+#define __S110	PAGE_EX_RW
+#define __S111	PAGE_EX_RW
+
+#ifndef __s390x__
+# define PMD_SHADOW_SHIFT	1
+# define PGD_SHADOW_SHIFT	1
+#else /* __s390x__ */
+# define PMD_SHADOW_SHIFT	2
+# define PGD_SHADOW_SHIFT	2
+#endif /* __s390x__ */
+
+static inline struct page *get_shadow_page(struct page *page)
+{
+	if (s390_noexec && !list_empty(&page->lru))
+		return virt_to_page(page->lru.next);
+	return NULL;
+}
+
+static inline pte_t *get_shadow_pte(pte_t *ptep)
+{
+	unsigned long pteptr = (unsigned long) (ptep);
+
+	if (s390_noexec) {
+		unsigned long offset = pteptr & (PAGE_SIZE - 1);
+		void *addr = (void *) (pteptr ^ offset);
+		struct page *page = virt_to_page(addr);
+		if (!list_empty(&page->lru))
+			return (pte_t *) ((unsigned long) page->lru.next |
+								offset);
+	}
+	return NULL;
+}
+
+static inline pmd_t *get_shadow_pmd(pmd_t *pmdp)
+{
+	unsigned long pmdptr = (unsigned long) (pmdp);
+
+	if (s390_noexec) {
+		unsigned long offset = pmdptr &
+				((PAGE_SIZE << PMD_SHADOW_SHIFT) - 1);
+		void *addr = (void *) (pmdptr ^ offset);
+		struct page *page = virt_to_page(addr);
+		if (!list_empty(&page->lru))
+			return (pmd_t *) ((unsigned long) page->lru.next |
+								offset);
+	}
+	return NULL;
+}
+
+static inline pgd_t *get_shadow_pgd(pgd_t *pgdp)
+{
+	unsigned long pgdptr = (unsigned long) (pgdp);
+
+	if (s390_noexec) {
+		unsigned long offset = pgdptr &
+				((PAGE_SIZE << PGD_SHADOW_SHIFT) - 1);
+		void *addr = (void *) (pgdptr ^ offset);
+		struct page *page = virt_to_page(addr);
+		if (!list_empty(&page->lru))
+			return (pgd_t *) ((unsigned long) page->lru.next |
+								offset);
+	}
+	return NULL;
+}
 
 /*
  * Certain architectures need to do special things when PTEs
@@ -348,7 +419,16 @@ extern unsigned long vmalloc_end;
  */
 static inline void set_pte(pte_t *pteptr, pte_t pteval)
 {
+	pte_t *shadow_pte = get_shadow_pte(pteptr);
+
 	*pteptr = pteval;
+	if (shadow_pte) {
+		if (!(pte_val(pteval) & _PAGE_INVALID) &&
+		    (pte_val(pteval) & _PAGE_SWX))
+			pte_val(*shadow_pte) = pte_val(pteval) | _PAGE_RO;
+		else
+			pte_val(*shadow_pte) = _PAGE_TYPE_EMPTY;
+	}
 }
 #define set_pte_at(mm,addr,ptep,pteval) set_pte(ptep,pteval)
 
@@ -466,7 +546,7 @@ static inline int pte_read(pte_t pte)
 
 static inline void pgd_clear(pgd_t * pgdp)      { }
 
-static inline void pmd_clear(pmd_t * pmdp)
+static inline void pmd_clear_kernel(pmd_t * pmdp)
 {
 	pmd_val(pmdp[0]) = _PAGE_TABLE_INV;
 	pmd_val(pmdp[1]) = _PAGE_TABLE_INV;
@@ -474,24 +554,55 @@ static inline void pmd_clear(pmd_t * pmdp)
 	pmd_val(pmdp[3]) = _PAGE_TABLE_INV;
 }
 
+static inline void pmd_clear(pmd_t * pmdp)
+{
+	pmd_t *shadow_pmd = get_shadow_pmd(pmdp);
+
+	pmd_clear_kernel(pmdp);
+	if (shadow_pmd)
+		pmd_clear_kernel(shadow_pmd);
+}
+
 #else /* __s390x__ */
 
-static inline void pgd_clear(pgd_t * pgdp)
+static inline void pgd_clear_kernel(pgd_t * pgdp)
 {
 	pgd_val(*pgdp) = _PGD_ENTRY_INV | _PGD_ENTRY;
 }
 
-static inline void pmd_clear(pmd_t * pmdp)
+static inline void pgd_clear(pgd_t * pgdp)
+{
+	pgd_t *shadow_pgd = get_shadow_pgd(pgdp);
+
+	pgd_clear_kernel(pgdp);
+	if (shadow_pgd)
+		pgd_clear_kernel(shadow_pgd);
+}
+
+static inline void pmd_clear_kernel(pmd_t * pmdp)
 {
 	pmd_val(*pmdp) = _PMD_ENTRY_INV | _PMD_ENTRY;
 	pmd_val1(*pmdp) = _PMD_ENTRY_INV | _PMD_ENTRY;
 }
 
+static inline void pmd_clear(pmd_t * pmdp)
+{
+	pmd_t *shadow_pmd = get_shadow_pmd(pmdp);
+
+	pmd_clear_kernel(pmdp);
+	if (shadow_pmd)
+		pmd_clear_kernel(shadow_pmd);
+}
+
 #endif /* __s390x__ */
 
 static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
 {
+	pte_t *shadow_pte = get_shadow_pte(ptep);
+
 	pte_val(*ptep) = _PAGE_TYPE_EMPTY;
+	if (shadow_pte)
+		pte_val(*shadow_pte) = _PAGE_TYPE_EMPTY;
 }
 
 /*
@@ -609,8 +720,11 @@ ptep_clear_flush(struct vm_area_struct *vma,
 		 unsigned long address, pte_t *ptep)
 {
 	pte_t pte = *ptep;
+	pte_t *shadow_pte = get_shadow_pte(ptep);
 
 	__ptep_ipte(address, ptep);
+	if (shadow_pte)
+		__ptep_ipte(address, shadow_pte);
 	return pte;
 }