[PATCH 3/3] lib/vsprintf: make-printk-non-secret printks all addresses as unhashed

Timur Tabi
2021-02-10T05:19:58Z

If the make-printk-non-secret command line parameter is set, then
printk("%p") will print pointers as unhashed. This is useful for
debugging purposes.

A large warning message is displayed if this option is enabled.
Unhashed pointers, while useful for debugging, 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>
---
.../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..6962379469e4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2613,6 +2613,21 @@
different yeeloong laptops.
Example: machtype=lemote-yeeloong-2f-7inch

+ make-printk-non-secret
+ Force pointers printed to the console to be unhashed.
+ By default, when a pointer is printed to the kernel
+ console (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. If this 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.
+
max_addr=nn[KMG] [KNL,BOOT,ia64] All physical memory greater
than or equal to this physical address is ignored.

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..1296d9b0b328 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("make-printk-non-secret", 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
+ * make-printk-non-secret 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