[PATCH 3/3] [v3] lib/vsprintf: debug_never_hash_pointers prints all addresses as unhashed

From: Timur Tabi <timur@kernel.org>
To: Petr Mladek <pmladek@suse.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	akpm@linux-foundation.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	roman.fietze@magna.com, Kees Cook <keescook@chromium.org>,
	John Ogness <john.ogness@linutronix.de>,
	akinobu.mita@gmail.com, glider@google.com,
	Andrey Konovalov <andreyknvl@google.com>,
	Marco Elver <elver@google.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Pavel Machek <pavel@ucw.cz>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: [PATCH 3/3] [v3] lib/vsprintf: debug_never_hash_pointers prints all addresses as unhashed
Date: Wed, 10 Feb 2021 15:34:53 -0600	[thread overview]
Message-ID: <20210210213453.1504219-4-timur@kernel.org> (raw)
In-Reply-To: <20210210213453.1504219-1-timur@kernel.org>

If the debug_never_hash_pointers command line parameter is set, then
printk("%p") will print pointers as unhashed, which is useful for
debugging purposes.  This also applies to any function that uses
vsprintf, such as print_hex_dump() and seq_buf_printf().

A large warning message is displayed if this option is enabled.
Unhashed pointers expose kernel addresses, which can be a security
risk.

Also update test_printf to skip the hashed pointer tests if the
command-line option is set.

Signed-off-by: Timur Tabi <timur@kernel.org>
Acked-by: Petr Mladek <pmladek@suse.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
---
 .../admin-guide/kernel-parameters.txt         | 15 ++++++++
 lib/test_printf.c                             |  8 ++++
 lib/vsprintf.c                                | 38 ++++++++++++++++++-
 3 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a10b545c2070..2a97e787f49c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -810,6 +810,21 @@
 			1 will print _a lot_ more information - normally
 			only useful to kernel developers.
 
+	debug_never_hash_pointers
+			Force pointers printed to the console or buffers to be
+			unhashed.  By default, when a pointer is printed via %p
+			format string, that pointer is "hashed", i.e. obscured
+			by hashing the pointer value.  This is a security feature
+			that hides actual kernel addresses from unprivileged
+			users, but it also makes debugging the kernel more
+			difficult since unequal pointers can no longer be
+			compared.  However, if this command-line option is
+			specified, then all normal pointers will have their true
+			value printed.  Pointers printed via %pK may still be
+			hashed.  This option should only be specified when
+			debugging the kernel.  Please do not use on production
+			kernels.
+
 	debug_objects	[KNL] Enable object debugging
 
 	no_debug_objects
diff --git a/lib/test_printf.c b/lib/test_printf.c
index ad2bcfa8caa1..b0b62d76e598 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -35,6 +35,8 @@ KSTM_MODULE_GLOBALS();
 static char *test_buffer __initdata;
 static char *alloced_buffer __initdata;
 
+extern bool debug_never_hash_pointers;
+
 static int __printf(4, 0) __init
 do_test(int bufsize, const char *expect, int elen,
 	const char *fmt, va_list ap)
@@ -301,6 +303,12 @@ plain(void)
 {
 	int err;
 
+	if (debug_never_hash_pointers) {
+		pr_warn("skipping plain 'p' tests");
+		skipped_tests += 2;
+		return;
+	}
+
 	err = plain_hash();
 	if (err) {
 		pr_warn("plain 'p' does not appear to be hashed\n");
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3b53c73580c5..b4e07ecb1cb2 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2090,6 +2090,34 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
 	return widen_string(buf, buf - buf_start, end, spec);
 }
 
+/* Disable pointer hashing if requested */
+bool debug_never_hash_pointers __ro_after_init;
+EXPORT_SYMBOL_GPL(debug_never_hash_pointers);
+
+static int __init debug_never_hash_pointers_enable(char *str)
+{
+	debug_never_hash_pointers = true;
+
+	pr_warn("**********************************************************\n");
+	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
+	pr_warn("**                                                      **\n");
+	pr_warn("** All pointers that are printed to the console will    **\n");
+	pr_warn("** be printed as unhashed.                              **\n");
+	pr_warn("**                                                      **\n");
+	pr_warn("** Kernel memory addresses are exposed, which may       **\n");
+	pr_warn("** reduce the security of your system.                  **\n");
+	pr_warn("**                                                      **\n");
+	pr_warn("** If you see this message and you are not debugging    **\n");
+	pr_warn("** the kernel, report this immediately to your system   **\n");
+	pr_warn("** administrator!                                       **\n");
+	pr_warn("**                                                      **\n");
+	pr_warn("**   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **\n");
+	pr_warn("**********************************************************\n");
+
+	return 0;
+}
+early_param("debug_never_hash_pointers", debug_never_hash_pointers_enable);
+
 /*
  * Show a '%p' thing.  A kernel extension is that the '%p' is followed
  * by an extra set of alphanumeric characters that are extended format
@@ -2297,8 +2325,14 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		}
 	}
 
-	/* default is to _not_ leak addresses, hash before printing */
-	return ptr_to_id(buf, end, ptr, spec);
+	/*
+	 * default is to _not_ leak addresses, so hash before printing,
+	 * unless debug_never_hash_pointers is specified on the command line.
+	 */
+	if (unlikely(debug_never_hash_pointers))
+		return pointer_string(buf, end, ptr, spec);
+	else
+		return ptr_to_id(buf, end, ptr, spec);
 }
 
 /*
-- 
2.25.1

