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Subject[PATCH] vsprintf: Fix memory barriers of ptr_key to have_filed_random_ptr_key

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

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!

Because nothing prevents CPU1 from loading ptr_key before loading
have_filled_random_ptr_key.

Note, I also do not see the reason to use smp_mb() instead of smp_wmb()
since we are only worried about the store of ptr_key with respect to
the store of have_filled_random_ptr_key.

Cc: stable@vger.kernel.org
Fixes: ad67b74d2469d ("printk: hash addresses printed with %p")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 30c0cb8cc9bc..e8a0b8e54bd3 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1680,7 +1680,7 @@ static void fill_random_ptr_key(struct random_ready_callback *unused)
* ptr_to_id() needs to see have_filled_random_ptr_key==true
* after get_random_bytes() returns.
*/
- smp_mb();
+ smp_wmb();
WRITE_ONCE(have_filled_random_ptr_key, true);
}

@@ -1715,6 +1715,9 @@ static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
return string(buf, end, "(ptrval)", spec);
}

+ /* Read ptr_key after reading have_filled_random_ptr_key */
+ smp_rmb();
+
#ifdef CONFIG_64BIT
hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
/*
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