x86/mm: Warn on W^X mappings

Warn on any residual W+X mappings after setting NX
if DEBUG_WX is enabled.  Introduce a separate
X86_PTDUMP_CORE config that enables the code for
dumping the page tables without enabling the debugfs
interface, so that DEBUG_WX can be enabled without
exposing the debugfs interface.  Switch EFI_PGT_DUMP
to using X86_PTDUMP_CORE so that it also does not require
enabling the debugfs interface.

On success it prints this to the kernel log:

  x86/mm: Checked W+X mappings: passed, no W+X pages found.

On failure it prints a warning and a count of the failed pages:

  ------------[ cut here ]------------
  WARNING: CPU: 1 PID: 1 at arch/x86/mm/dump_pagetables.c:226 note_page+0x610/0x7b0()
  x86/mm: Found insecure W+X mapping at address ffffffff81755000/__stop___ex_table+0xfa8/0xabfa8
  [...]
  Call Trace:
   [<ffffffff81380a5f>] dump_stack+0x44/0x55
   [<ffffffff8109d3f2>] warn_slowpath_common+0x82/0xc0
   [<ffffffff8109d48c>] warn_slowpath_fmt+0x5c/0x80
   [<ffffffff8106cfc9>] ? note_page+0x5c9/0x7b0
   [<ffffffff8106d010>] note_page+0x610/0x7b0
   [<ffffffff8106d409>] ptdump_walk_pgd_level_core+0x259/0x3c0
   [<ffffffff8106d5a7>] ptdump_walk_pgd_level_checkwx+0x17/0x20
   [<ffffffff81063905>] mark_rodata_ro+0xf5/0x100
   [<ffffffff817415a0>] ? rest_init+0x80/0x80
   [<ffffffff817415bd>] kernel_init+0x1d/0xe0
   [<ffffffff8174cd1f>] ret_from_fork+0x3f/0x70
   [<ffffffff817415a0>] ? rest_init+0x80/0x80
  ---[ end trace a1f23a1e42a2ac76 ]---
  x86/mm: Checked W+X mappings: FAILED, 171 W+X pages found.

Signed-off-by: Stephen Smalley <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
[ Improved the Kconfig help text and made the new option default-y
  if CONFIG_DEBUG_RODATA=y, because it already found buggy mappings,
  so we really want people to have this on by default. ]
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/Kconfig.debug

@@ -65,10 +65,14 @@ config EARLY_PRINTK_EFI
 	  This is useful for kernel debugging when your machine crashes very
 	  early before the console code is initialized.
 
+config X86_PTDUMP_CORE
+	def_bool n
+
 config X86_PTDUMP
 	bool "Export kernel pagetable layout to userspace via debugfs"
 	depends on DEBUG_KERNEL
 	select DEBUG_FS
+	select X86_PTDUMP_CORE
 	---help---
 	  Say Y here if you want to show the kernel pagetable layout in a
 	  debugfs file. This information is only useful for kernel developers
@@ -79,7 +83,8 @@ config X86_PTDUMP
 
 config EFI_PGT_DUMP
 	bool "Dump the EFI pagetable"
-	depends on EFI && X86_PTDUMP
+	depends on EFI
+	select X86_PTDUMP_CORE
 	---help---
 	  Enable this if you want to dump the EFI page table before
 	  enabling virtual mode. This can be used to debug miscellaneous
@@ -105,6 +110,35 @@ config DEBUG_RODATA_TEST
 	  feature as well as for the change_page_attr() infrastructure.
 	  If in doubt, say "N"
 
+config DEBUG_WX
+	bool "Warn on W+X mappings at boot"
+	depends on DEBUG_RODATA
+	default y
+	select X86_PTDUMP_CORE
+	---help---
+	  Generate a warning if any W+X mappings are found at boot.
+
+	  This is useful for discovering cases where the kernel is leaving
+	  W+X mappings after applying NX, as such mappings are a security risk.
+
+	  Look for a message in dmesg output like this:
+
+	    x86/mm: Checked W+X mappings: passed, no W+X pages found.
+
+	  or like this, if the check failed:
+
+	    x86/mm: Checked W+X mappings: FAILED, <N> W+X pages found.
+
+	  Note that even if the check fails, your kernel is possibly
+	  still fine, as W+X mappings are not a security hole in
+	  themselves, what they do is that they make the exploitation
+	  of other unfixed kernel bugs easier.
+
+	  There is no runtime or memory usage effect of this option
+	  once the kernel has booted up - it's a one time check.
+
+	  If in doubt, say "Y".
+
 config DEBUG_SET_MODULE_RONX
 	bool "Set loadable kernel module data as NX and text as RO"
 	depends on MODULES

arch/x86/include/asm/pgtable.h

@@ -19,6 +19,13 @@
 #include <asm/x86_init.h>
 
 void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd);
+void ptdump_walk_pgd_level_checkwx(void);
+
+#ifdef CONFIG_DEBUG_WX
+#define debug_checkwx() ptdump_walk_pgd_level_checkwx()
+#else
+#define debug_checkwx() do { } while (0)
+#endif
 
 /*
  * ZERO_PAGE is a global shared page that is always zero: used

arch/x86/mm/Makefile

@@ -14,7 +14,7 @@ obj-$(CONFIG_SMP)		+= tlb.o
 obj-$(CONFIG_X86_32)		+= pgtable_32.o iomap_32.o
 
 obj-$(CONFIG_HUGETLB_PAGE)	+= hugetlbpage.o
-obj-$(CONFIG_X86_PTDUMP)	+= dump_pagetables.o
+obj-$(CONFIG_X86_PTDUMP_CORE)	+= dump_pagetables.o
 
 obj-$(CONFIG_HIGHMEM)		+= highmem_32.o
 

arch/x86/mm/dump_pagetables.c

@@ -32,6 +32,8 @@ struct pg_state {
 	const struct addr_marker *marker;
 	unsigned long lines;
 	bool to_dmesg;
+	bool check_wx;
+	unsigned long wx_pages;
 };
 
 struct addr_marker {
@@ -214,6 +216,16 @@ static void note_page(struct seq_file *m, struct pg_state *st,
 		const char *unit = units;
 		unsigned long delta;
 		int width = sizeof(unsigned long) * 2;
+		pgprotval_t pr = pgprot_val(st->current_prot);
+
+		if (st->check_wx && (pr & _PAGE_RW) && !(pr & _PAGE_NX)) {
+			WARN_ONCE(1,
+				  "x86/mm: Found insecure W+X mapping at address %p/%pS\n",
+				  (void *)st->start_address,
+				  (void *)st->start_address);
+			st->wx_pages += (st->current_address -
+					 st->start_address) / PAGE_SIZE;
+		}
 
 		/*
 		 * Now print the actual finished series
@@ -346,7 +358,8 @@ static void walk_pud_level(struct seq_file *m, struct pg_state *st, pgd_t addr,
 #define pgd_none(a)  pud_none(__pud(pgd_val(a)))
 #endif
 
-void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
+static void ptdump_walk_pgd_level_core(struct seq_file *m, pgd_t *pgd,
+				       bool checkwx)
 {
 #ifdef CONFIG_X86_64
 	pgd_t *start = (pgd_t *) &init_level4_pgt;
@@ -362,6 +375,10 @@ void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
 		st.to_dmesg = true;
 	}
 
+	st.check_wx = checkwx;
+	if (checkwx)
+		st.wx_pages = 0;
+
 	for (i = 0; i < PTRS_PER_PGD; i++) {
 		st.current_address = normalize_addr(i * PGD_LEVEL_MULT);
 		if (!pgd_none(*start)) {
@@ -381,8 +398,26 @@ void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
 	/* Flush out the last page */
 	st.current_address = normalize_addr(PTRS_PER_PGD*PGD_LEVEL_MULT);
 	note_page(m, &st, __pgprot(0), 0);
+	if (!checkwx)
+		return;
+	if (st.wx_pages)
+		pr_info("x86/mm: Checked W+X mappings: FAILED, %lu W+X pages found.\n",
+			st.wx_pages);
+	else
+		pr_info("x86/mm: Checked W+X mappings: passed, no W+X pages found.\n");
+}
+
+void ptdump_walk_pgd_level(struct seq_file *m, pgd_t *pgd)
+{
+	ptdump_walk_pgd_level_core(m, pgd, false);
 }
 
+void ptdump_walk_pgd_level_checkwx(void)
+{
+	ptdump_walk_pgd_level_core(NULL, NULL, true);
+}
+
+#ifdef CONFIG_X86_PTDUMP
 static int ptdump_show(struct seq_file *m, void *v)
 {
 	ptdump_walk_pgd_level(m, NULL);
@@ -400,10 +435,13 @@ static const struct file_operations ptdump_fops = {
 	.llseek		= seq_lseek,
 	.release	= single_release,
 };
+#endif
 
 static int pt_dump_init(void)
 {
+#ifdef CONFIG_X86_PTDUMP
 	struct dentry *pe;
+#endif
 
 #ifdef CONFIG_X86_32
 	/* Not a compile-time constant on x86-32 */
@@ -415,10 +453,12 @@ static int pt_dump_init(void)
 	address_markers[FIXADDR_START_NR].start_address = FIXADDR_START;
 #endif
 
+#ifdef CONFIG_X86_PTDUMP
 	pe = debugfs_create_file("kernel_page_tables", 0600, NULL, NULL,
 				 &ptdump_fops);
 	if (!pe)
 		return -ENOMEM;
+#endif
 
 	return 0;
 }

arch/x86/mm/init_32.c

@@ -957,6 +957,8 @@ void mark_rodata_ro(void)
 	set_pages_ro(virt_to_page(start), size >> PAGE_SHIFT);
 #endif
 	mark_nxdata_nx();
+	if (__supported_pte_mask & _PAGE_NX)
+		debug_checkwx();
 }
 #endif
 

arch/x86/mm/init_64.c

@@ -1150,6 +1150,8 @@ void mark_rodata_ro(void)
 	free_init_pages("unused kernel",
 			(unsigned long) __va(__pa_symbol(rodata_end)),
 			(unsigned long) __va(__pa_symbol(_sdata)));
+
+	debug_checkwx();
 }
 
 #endif