[PATCH v2] tracing, printk: Force no hashing when trace_printk() is used

[Steven Rostedt]

From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

While debugging an issue I needed to see if the pointers were being
processed correctly with trace_printk() and after using "%p" and
triggering my bug and trace output, I was disappointed that all my
pointers were random garbage and didn't produce anything useful for me.
I had to rewrite all the trace_printk()s to use "%lx" instead.

As trace_printk() is not to be used for anything but debugging, and
this is enforced by printing in the dmesg:

 **********************************************************
 **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
 **                                                      **
 ** trace_printk() being used. Allocating extra memory.  **
 **                                                      **
 ** This means that this is a DEBUG kernel and it is     **
 ** unsafe for production use.                           **
 **                                                      **
 ** If you see this message and you are not debugging    **
 ** the kernel, report this immediately to your vendor!  **
 **                                                      **
 **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
 **********************************************************

on boot up if trace_printk() is used (or when a module is loaded that
uses trace_printk()), we can safely assume that the use of
trace_printk() is not going to be accidentally added to production code
(and if it is, they should be whacked with an overcooked spaghetti
noodle).

A static_key is added called "trace_debug" and if it is set, then %p will
not be hashed.

Both trace_debug is set and kptr_restrict is set to zero in the same
code that produces the above banner. This will allow trace_printk() to
not be affected by security code, as trace_printk() should never be run
on a machine that needs security of this kind.

Link: http://lkml.kernel.org/r/20180403154102.150b1be0@xxxxxxxxxxxxxxxxxx

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
 include/linux/printk.h | 1 +
 kernel/trace/trace.c   | 4 ++++
 lib/vsprintf.c         | 5 +++++
 3 files changed, 10 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index e9b603ee9953..b624493b3991 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -278,6 +278,7 @@ static inline void printk_safe_flush_on_panic(void)
 #endif
 
 extern int kptr_restrict;
+extern struct static_key trace_debug;
 
 extern asmlinkage void dump_stack(void) __cold;
 
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 0f47e653ffd8..6c151d00848b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2864,6 +2864,10 @@ void trace_printk_init_buffers(void)
 
 	buffers_allocated = 1;
 
+	/* This is a debug kernel, allow pointers to be shown */
+	static_key_enable(&trace_debug);
+	kptr_restrict = 0;
+
 	/*
 	 * trace_printk_init_buffers() can be called by modules.
 	 * If that happens, then we need to start cmdline recording
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 89f8a4a4b770..c3d8eafecb39 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1345,6 +1345,7 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
 }
 
 int kptr_restrict __read_mostly;
+struct static_key trace_debug = STATIC_KEY_INIT_FALSE;
 
 static noinline_for_stack
 char *restricted_pointer(char *buf, char *end, const void *ptr,
@@ -1962,6 +1963,10 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		return pointer_string(buf, end, ptr, spec);
 	}
 
+	/* When the kernel is in debugging mode, show all pointers */
+	if (static_key_false(&trace_debug))
+		return restricted_pointer(buf, end, ptr, spec);
+
 	/* default is to _not_ leak addresses, hash before printing */
 	return ptr_to_id(buf, end, ptr, spec);
 }
-- 
2.13.6