From: Steven Rostedt (VMware) <[email protected]>
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: [email protected]
Fixes: ad67b74d2469d ("printk: hash addresses printed with %p")
Signed-off-by: Steven Rostedt (VMware) <[email protected]>
---
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);
/*
