mm/init: Add 'rodata=off' boot cmdline parameter to disable read-only kernel mappings

It may be useful to debug writes to the readonly sections of memory,
so provide a cmdline "rodata=off" to allow for this. This can be
expanded in the future to support "log" and "write" modes, but that
will need to be architecture-specific.

This also makes KDB software breakpoints more usable, as read-only
mappings can now be disabled on any kernel.

Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Brown <david.brown@linaro.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel-hardening@lists.openwall.com
Cc: linux-arch <linux-arch@vger.kernel.org>
Link: http://lkml.kernel.org/r/1455748879-21872-3-git-send-email-keescook@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 9a53c92..0003367 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3491,6 +3491,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
        ro              [KNL] Mount root device read-only on boot
 
+       rodata=         [KNL]
+               on      Mark read-only kernel memory as read-only (default).
+               off     Leave read-only kernel memory writable for debugging.
+
        root=           [KNL] Root filesystem
                        See name_to_dev_t comment in init/do_mounts.c.
 

diff --git a/init/main.c b/init/main.c
index 58c9e37..928a343 100644 (file)
--- a/init/main.c
+++ b/init/main.c
@@ -93,9 +93,6 @@ static int kernel_init(void *);
 extern void init_IRQ(void);
 extern void fork_init(void);
 extern void radix_tree_init(void);
-#ifndef CONFIG_DEBUG_RODATA
-static inline void mark_rodata_ro(void) { }
-#endif
 
 /*
  * Debug helper: via this flag we know that we are in 'early bootup code'
@@ -929,6 +926,28 @@ static int try_to_run_init_process(const char *init_filename)
 
 static noinline void __init kernel_init_freeable(void);
 
+#ifdef CONFIG_DEBUG_RODATA
+static bool rodata_enabled = true;
+static int __init set_debug_rodata(char *str)
+{
+       return strtobool(str, &rodata_enabled);
+}
+__setup("rodata=", set_debug_rodata);
+
+static void mark_readonly(void)
+{
+       if (rodata_enabled)
+               mark_rodata_ro();
+       else
+               pr_info("Kernel memory protection disabled.\n");
+}
+#else
+static inline void mark_readonly(void)
+{
+       pr_warn("This architecture does not have kernel memory protection.\n");
+}
+#endif
+
 static int __ref kernel_init(void *unused)
 {
        int ret;
@@ -937,7 +956,7 @@ static int __ref kernel_init(void *unused)
        /* need to finish all async __init code before freeing the memory */
        async_synchronize_full();
        free_initmem();
-       mark_rodata_ro();
+       mark_readonly();
        system_state = SYSTEM_RUNNING;
        numa_default_policy();
 

diff --git a/kernel/debug/kdb/kdb_bp.c b/kernel/debug/kdb/kdb_bp.c
index e1dbf4a..90ff129 100644 (file)
--- a/kernel/debug/kdb/kdb_bp.c
+++ b/kernel/debug/kdb/kdb_bp.c
@@ -153,13 +153,11 @@ static int _kdb_bp_install(struct pt_regs *regs, kdb_bp_t *bp)
        } else {
                kdb_printf("%s: failed to set breakpoint at 0x%lx\n",
                           __func__, bp->bp_addr);
-#ifdef CONFIG_DEBUG_RODATA
                if (!bp->bp_type) {
                        kdb_printf("Software breakpoints are unavailable.\n"
-                                  "  Change the kernel CONFIG_DEBUG_RODATA=n\n"
+                                  "  Boot the kernel with rodata=off\n"
                                   "  OR use hw breaks: help bph\n");
                }
-#endif
                return 1;
        }
        return 0;