Commit 85f4f12d authored May 15, 2018 by Steven Rostedt (VMware)

vsprintf: Replace memory barrier with static_key for random_ptr_key update

Reviewing Tobin's patches for getting pointers out early before
entropy has been established, I noticed that there's a lone smp_mb() in
the code. As with most lone memory barriers, this one appears to be
incorrectly used.

We currently basically have this:

	get_random_bytes(&ptr_key, sizeof(ptr_key));
	/*
	 * have_filled_random_ptr_key==true is dependent on get_random_bytes().
	 * ptr_to_id() needs to see have_filled_random_ptr_key==true
	 * after get_random_bytes() returns.
	 */
	smp_mb();
	WRITE_ONCE(have_filled_random_ptr_key, true);

And later we have:

	if (unlikely(!have_filled_random_ptr_key))
		return string(buf, end, "(ptrval)", spec);

/* Missing memory barrier here. */

	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);

As the CPU can perform speculative loads, we could have a situation
with the following:

	CPU0				CPU1
	----				----
				   load ptr_key = 0
   store ptr_key = random
   smp_mb()
   store have_filled_random_ptr_key

				   load have_filled_random_ptr_key = true

				    BAD BAD BAD! (you're so bad!)

Because nothing prevents CPU1 from loading ptr_key before loading
have_filled_random_ptr_key.

But this race is very unlikely, but we can't keep an incorrect smp_mb() in
place. Instead, replace the have_filled_random_ptr_key with a static_branch
not_filled_random_ptr_key, that is initialized to true and changed to false
when we get enough entropy. If the update happens in early boot, the
static_key is updated immediately, otherwise it will have to wait till
entropy is filled and this happens in an interrupt handler which can't
enable a static_key, as that requires a preemptible context. In that case, a
work_queue is used to enable it, as entropy already took too long to
establish in the first place waiting a little more shouldn't hurt anything.

The benefit of using the static key is that the unlikely branch in
vsprintf() now becomes a nop.

Link: http://lkml.kernel.org/r/20180515100558.21df515e@gandalf.local.home



Cc: stable@vger.kernel.org
Fixes: ad67b74d ("printk: hash addresses printed with %p")
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

parent 67b8d5c7

lib/vsprintf.c

+15 −11

Original line number	Diff line number	Diff line
		@@ -1669,19 +1669,22 @@ char pointer_string(char buf, char end, const void ptr,
		return number(buf, end, (unsigned long int)ptr, spec);
		}

		static bool have_filled_random_ptr_key __read_mostly;
		static DEFINE_STATIC_KEY_TRUE(not_filled_random_ptr_key);
		static siphash_key_t ptr_key __read_mostly;

		static void fill_random_ptr_key(struct random_ready_callback *unused)
		static void enable_ptr_key_workfn(struct work_struct *work)
		{
		get_random_bytes(&ptr_key, sizeof(ptr_key));
		/*
		* have_filled_random_ptr_key==true is dependent on get_random_bytes().
		* ptr_to_id() needs to see have_filled_random_ptr_key==true
		* after get_random_bytes() returns.
		*/
		smp_mb();
		WRITE_ONCE(have_filled_random_ptr_key, true);
		/* Needs to run from preemptible context */
		static_branch_disable(&not_filled_random_ptr_key);
		}

		static DECLARE_WORK(enable_ptr_key_work, enable_ptr_key_workfn);

		static void fill_random_ptr_key(struct random_ready_callback *unused)
		{
		/* This may be in an interrupt handler. */
		queue_work(system_unbound_wq, &enable_ptr_key_work);
		}

		static struct random_ready_callback random_ready = {
		@@ -1695,7 +1698,8 @@ static int __init initialize_ptr_random(void)
		if (!ret) {
		return 0;
		} else if (ret == -EALREADY) {
		fill_random_ptr_key(&random_ready);
		/* This is in preemptible context */
		enable_ptr_key_workfn(&enable_ptr_key_work);
		return 0;
		}

		@@ -1709,7 +1713,7 @@ static char ptr_to_id(char buf, char end, void ptr, struct printf_spec spec)
		unsigned long hashval;
		const int default_width = 2 * sizeof(ptr);

		if (unlikely(!have_filled_random_ptr_key)) {
		if (static_branch_unlikely(&not_filled_random_ptr_key)) {
		spec.field_width = default_width;
		/* string length must be less than default_width */
		return string(buf, end, "(ptrval)", spec);

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