[PATCH v10 06/10] powerpc/mm/ptdump: debugfs handler for W+X checks at runtime

[Jordan Niethe]

From: Russell Currey <ruscur at russell.cc>

Optionally run W+X checks when dumping pagetable information to
debugfs' kernel_page_tables.

To use:
    $ echo 1 > /sys/kernel/debug/check_wx_pages
    $ cat /sys/kernel/debug/kernel_page_tables

and check the kernel log.  Useful for testing strict module RWX.

To disable W+X checks:
	$ echo 0 > /sys/kernel/debug/check_wx_pages

Update the Kconfig entry to reflect this.

Also fix a typo.

Reviewed-by: Kees Cook <keescook at chromium.org>
Signed-off-by: Russell Currey <ruscur at russell.cc>
[jpn: Change check_wx_pages to act as mode bit affecting
      kernel_page_tables instead of triggering action on its own]
Signed-off-by: Jordan Niethe <jniethe5 at gmail.com>
---
v10: check_wx_pages now affects kernel_page_tables rather then triggers
     its own action.
---
 arch/powerpc/Kconfig.debug      |  6 ++++--
 arch/powerpc/mm/ptdump/ptdump.c | 34 ++++++++++++++++++++++++++++++++-
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index ae084357994e..56e99e9a30d9 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug
@@ -371,7 +371,7 @@ config PPC_PTDUMP
 	  If you are unsure, say N.
 
 config PPC_DEBUG_WX
-	bool "Warn on W+X mappings at boot"
+	bool "Warn on W+X mappings at boot & enable manual checks at runtime"
 	depends on PPC_PTDUMP && STRICT_KERNEL_RWX
 	help
 	  Generate a warning if any W+X mappings are found at boot.
@@ -385,7 +385,9 @@ config PPC_DEBUG_WX
 	  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.
+	  once the kernel has booted up, it only automatically checks once.
+
+	  Enables the "check_wx_pages" debugfs entry for checking at runtime.
 
 	  If in doubt, say "Y".
 
diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
index aca354fb670b..6592f7a48c96 100644
--- a/arch/powerpc/mm/ptdump/ptdump.c
+++ b/arch/powerpc/mm/ptdump/ptdump.c
@@ -4,7 +4,7 @@
  *
  * This traverses the kernel pagetables and dumps the
  * information about the used sections of memory to
- * /sys/kernel/debug/kernel_pagetables.
+ * /sys/kernel/debug/kernel_page_tables.
  *
  * Derived from the arm64 implementation:
  * Copyright (c) 2014, The Linux Foundation, Laura Abbott.
@@ -27,6 +27,8 @@
 
 #include "ptdump.h"
 
+static bool check_wx;
+
 /*
  * To visualise what is happening,
  *
@@ -410,6 +412,9 @@ static int ptdump_show(struct seq_file *m, void *v)
 	/* Traverse kernel page tables */
 	walk_pagetables(&st);
 	note_page(&st, 0, 0, 0, 0);
+
+	if (check_wx)
+		ptdump_check_wx();
 	return 0;
 }
 
@@ -459,6 +464,33 @@ void ptdump_check_wx(void)
 	else
 		pr_info("Checked W+X mappings: passed, no W+X pages found\n");
 }
+
+static int check_wx_debugfs_set(void *data, u64 val)
+{
+	if (val == 1ULL)
+		check_wx = true;
+	else if (val == 0ULL)
+		check_wx = false;
+	else
+		return -EINVAL;
+
+	return 0;
+}
+
+static int check_wx_debugfs_get(void *data, u64 *val)
+{
+	*val = check_wx ? 1 : 0;
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(check_wx_fops, check_wx_debugfs_get, check_wx_debugfs_set, "%llu\n");
+
+static int ptdump_check_wx_init(void)
+{
+	return debugfs_create_file("check_wx_pages", 0200, NULL,
+				   NULL, &check_wx_fops) ? 0 : -ENOMEM;
+}
+device_initcall(ptdump_check_wx_init);
 #endif
 
 static int ptdump_init(void)
-- 
2.25.1