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Message-Id: <20210210213453.1504219-4-timur@kernel.org>
Date: Wed, 10 Feb 2021 15:34:53 -0600
From: Timur Tabi <timur@...nel.org>
To: Petr Mladek <pmladek@...e.com>,
Steven Rostedt <rostedt@...dmis.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Vlastimil Babka <vbabka@...e.cz>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
akpm@...ux-foundation.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
roman.fietze@...na.com, Kees Cook <keescook@...omium.org>,
John Ogness <john.ogness@...utronix.de>,
akinobu.mita@...il.com, glider@...gle.com,
Andrey Konovalov <andreyknvl@...gle.com>,
Marco Elver <elver@...gle.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Pavel Machek <pavel@....cz>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 3/3] [v3] lib/vsprintf: debug_never_hash_pointers prints all addresses as unhashed
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@...nel.org>
Acked-by: Petr Mladek <pmladek@...e.com>
Acked-by: Randy Dunlap <rdunlap@...radead.org>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Acked-by: Vlastimil Babka <vbabka@...e.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
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