[metal] Allow more fine-grained control over page permissions (#663)

- use PAGE_RSRV bit (originally only for blinkenlights),
  rather than PAGE_V bit, to indicate that a virtual address
  page has been reserved — this should allow a program to
  create & reserve inaccessible "guard pages"
- mark page table entries for non-code pages with PAGE_XD bit,
  which should be supported on (circa) post-2004 x86-64 CPUs

ape/ape.S

@@ -1285,8 +1285,8 @@ lcheck:	pushf				# check for i8086 / i8088 / i80186
 	jl	10f
 	mov	%edi,%eax
 	cpuid
-	mov	$1<<29,%edi		# need nexgen32e long mode support
-	and	%edi,%edx
+	mov	$1<<29|1<<20,%edi	# need nexgen32e long mode support
+	and	%edi,%edx		# & nx support
 	cmp	%edi,%edx
 	jne	10f
 	xor	%ax,%ax
@@ -1415,7 +1415,7 @@ pinit:	push	%ds
 	movl	$0x79000+PAGE_V+PAGE_RW,0x7b000-SEG	# PD←PDT     (+)
 	movl	$0x79000+PAGE_V+PAGE_RW,0x7a000-SEG	# PD←PDT     (-)
 	mov	$512,%cx				# PD±2MB
-	mov	$PAGE_V+PAGE_RW,%eax
+	mov	$PAGE_V+PAGE_RSRV+PAGE_RW,%eax
 	xor	%di,%di
 0:	stosl
 	add	$0x1000,%eax
@@ -1438,7 +1438,7 @@ golong:	cli
 	mov	%eax,%cr4
 	movl	$EFER,%ecx
 	rdmsr
-	or	$EFER_LME|EFER_SCE,%eax
+	or	$EFER_LME|EFER_SCE|EFER_NXE,%eax
 	wrmsr
 	lgdt	REAL(_gdtrphy)
 	mov	%cr0,%eax

libc/intrin/directmap-metal.c

@@ -43,7 +43,7 @@ noasan struct DirectMap sys_mmap_metal(void *paddr, size_t size, int prot,
       addr = 4096;
     for (i = 0; i < size; i += 4096) {
       pte = __get_virtual(mm, pml4t, addr + i, false);
-      if (pte && (*pte & PAGE_V)) {
+      if (pte && (*pte & (PAGE_V | PAGE_RSRV))) {
         addr = MAX(addr, sys_mmap_metal_break) + i + 4096;
         i = 0;
       }
@@ -55,7 +55,13 @@ noasan struct DirectMap sys_mmap_metal(void *paddr, size_t size, int prot,
     pte = __get_virtual(mm, pml4t, addr + i, true);
     if (pte && page) {
       __clear_page(BANE + page);
-      *pte = page | ((prot & PROT_WRITE) ? PAGE_RW : 0) | PAGE_U | PAGE_V;
+      page |= PAGE_RSRV | PAGE_U;
+      if ((prot & PROT_WRITE))
+        page |= PAGE_V | PAGE_RW;
+      else if ((prot & (PROT_READ | PROT_EXEC)))
+        page |= PAGE_V;
+      if (!(prot & PROT_EXEC)) page |= PAGE_XD;
+      *pte = page;
     } else {
       addr = -1;
       break;

libc/intrin/mman.greg.c

@@ -71,7 +71,7 @@ noasan textreal uint64_t *__get_virtual(struct mman *mm, uint64_t *t,
   for (h = 39;; h -= 9) {
     e = t + ((vaddr >> h) & 511);
     if (h == 12) return e;
-    if (!(*e & PAGE_V)) {
+    if (!(*e & (PAGE_V | PAGE_RSRV))) {
       if (!maketables) return NULL;
       if (!(p = __new_page(mm))) return NULL;
       __clear_page(BANE + p);
@@ -123,8 +123,8 @@ noasan textreal void __invert_memory_area(struct mman *mm, uint64_t *pml4t,
   pe = ROUNDUP(pe, 4096);
   for (p = ps; p != pe; p += 4096) {
     m = __get_virtual(mm, pml4t, BANE + p, true);
-    if (m && !(*m & PAGE_V)) {
-      *m = p | PAGE_V | pte_flags;
+    if (m && !(*m & (PAGE_V | PAGE_RSRV))) {
+      *m = p | PAGE_V | PAGE_RSRV | pte_flags;
     }
   }
 }
@@ -139,7 +139,7 @@ static noasan textreal void __invert_memory(struct mman *mm, uint64_t *pml4t) {
     /* ape/ape.S has already mapped the first 2 MiB of physical memory. */
     if (ps < 0x200000 && ps + size <= 0x200000)
       continue;
-    __invert_memory_area(mm, pml4t, ps, size, PAGE_RW);
+    __invert_memory_area(mm, pml4t, ps, size, PAGE_RW | PAGE_XD);
   }
 }
 
@@ -187,8 +187,12 @@ noasan textreal void __map_phdrs(struct mman *mm, uint64_t *pml4t, uint64_t b) {
   for (p = (struct Elf64_Phdr *)REAL(ape_phdrs), m = 0;
        p < (struct Elf64_Phdr *)REAL(ape_phdrs_end); ++p) {
     if (p->p_type == PT_LOAD || p->p_type == PT_GNU_STACK) {
-      f = PAGE_V | PAGE_U;
-      if (p->p_flags & PF_W) f |= PAGE_RW;
+      f = PAGE_RSRV | PAGE_U;
+      if (p->p_flags & PF_W)
+        f |= PAGE_V | PAGE_RW;
+      else if (p->p_flags & (PF_R | PF_X))
+        f |= PAGE_V;
+      if (!(p->p_flags & PF_X)) f |= PAGE_XD;
       for (i = 0; i < p->p_memsz; i += 4096) {
         if (i < p->p_filesz) {
           v = b + p->p_offset + i;

libc/runtime/efimain.greg.c

@@ -126,7 +126,7 @@ __msabi noasan EFI_STATUS EfiMain(EFI_HANDLE ImageHandle,
   pdpt2 = (uint64_t *)0x7c000;
   pml4t = (uint64_t *)0x7e000;
   for (i = 0; i < 512; ++i) {
-    pd[i] = 0x1000 * i + PAGE_V + PAGE_RW;
+    pd[i] = 0x1000 * i + PAGE_V + PAGE_RSRV + PAGE_RW;
   }
   pdt1[0] = (intptr_t)pd + PAGE_V + PAGE_RW;
   pdt2[0] = (intptr_t)pd + PAGE_V + PAGE_RW;

libc/vga/vga-init.greg.c

@@ -51,7 +51,8 @@ void _vga_reinit(struct Tty *tty, unsigned short starty, unsigned short startx,
     chr_ht = VGA_ASSUME_CHAR_HEIGHT_PX;
   chr_wid = VGA_ASSUME_CHAR_WIDTH_PX;
   /* Make sure the video buffer is mapped into virtual memory. */
-  __invert_memory_area(mm, __get_pml4t(), vid_buf_phy, vid_buf_sz, PAGE_RW);
+  __invert_memory_area(mm, __get_pml4t(), vid_buf_phy, vid_buf_sz,
+                       PAGE_RW | PAGE_XD);
   /*
    * Initialize our tty structure from the current screen geometry, screen
    * contents, cursor position, & character dimensions.