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Subject[PATCH 4.16 023/110] vsprintf: Replace memory barrier with static_key for random_ptr_key update
Date
4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

commit 85f4f12d51397f1648e1f4350f77e24039b82d61 upstream.

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: ad67b74d2469d ("printk: hash addresses printed with %p")
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
lib/vsprintf.c | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)

--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1659,19 +1659,22 @@ char *pointer_string(char *buf, char *en
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 = {
@@ -1685,7 +1688,8 @@ static int __init initialize_ptr_random(
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;
}

@@ -1699,7 +1703,7 @@ static char *ptr_to_id(char *buf, char *
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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