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

Timur Tabi
2021-02-02T20:19:02Z

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

A large warning message is displayed if this option is enabled,
because unhashed addresses, while useful for debugging, exposes
kernel addresses which can be a security risk.

Signed-off-by: Timur Tabi <timur@kernel.org>
---
lib/vsprintf.c | 34 ++++++++++++++++++++++++++++++++--
1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3b53c73580c5..b9f87084afb0 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2090,6 +2090,30 @@ 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 */
+static bool debug_never_hash_pointers __ro_after_init;
+
+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("** compromise security on 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 vendor! **\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 +2321,14 @@ 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