[PATCH 3/3] [v4] lib/vsprintf: no_hash_pointers prints all addresses as unhashed

Timur Tabi
2021-02-14T16:14:46Z

If the no_hash_pointers command line parameter is set, then
printk("%p") will print pointers as unhashed, which is useful for
debugging purposes. This change 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>
Acked-by: Marco Elver <elver@google.com>
---
.../admin-guide/kernel-parameters.txt | 15 ++++++++
lib/test_printf.c | 8 +++++
lib/vsprintf.c | 36 +++++++++++++++++--
3 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a10b545c2070..c8993a296e71 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3281,6 +3281,21 @@
in certain environments such as networked servers or
real-time systems.

+ no_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.
+
nohibernate [HIBERNATION] Disable hibernation and resume.

nohz= [KNL] Boottime enable/disable dynamic ticks
diff --git a/lib/test_printf.c b/lib/test_printf.c
index ad2bcfa8caa1..a6755798e9e6 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 no_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 (no_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..41ddc353ebb8 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2090,6 +2090,32 @@ 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 no_hash_pointers __ro_after_init;
+EXPORT_SYMBOL_GPL(no_hash_pointers);
+
+static int __init no_hash_pointers_enable(char *str)
+{
+ no_hash_pointers = true;
+
+ pr_warn("**********************************************************\n");
+ pr_warn("** NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE **\n");
+ pr_warn("** **\n");
+ pr_warn("** This system shows unhashed kernel memory addresses **\n");
+ pr_warn("** via the console, logs, and other interfaces. This **\n");
+ pr_warn("** might 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("no_hash_pointers", no_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 +2323,14 @@ 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 no_hash_pointers is specified on the command line.
+ */
+ if (unlikely(no_hash_pointers))
+ return pointer_string(buf, end, ptr, spec);
+ else
+ return ptr_to_id(buf, end, ptr, spec);
}

/*
--
2.25.1