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authorSteven Rostedt (VMware) <rostedt@goodmis.org>2018-05-16 05:24:52 +0300
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2018-05-16 16:01:41 +0300
commit85f4f12d51397f1648e1f4350f77e24039b82d61 (patch)
tree7913a89f385849f276a99c2fbfa2a1e345d3ddbd /lib/vsprintf.c
parent67b8d5c7081221efa252e111cd52532ec6d4266f (diff)
downloadlinux-85f4f12d51397f1648e1f4350f77e24039b82d61.tar.xz
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: ad67b74d2469d ("printk: hash addresses printed with %p") Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Diffstat (limited to 'lib/vsprintf.c')
-rw-r--r--lib/vsprintf.c26
1 files changed, 15 insertions, 11 deletions
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 30c0cb8cc9bc..23920c5ff728 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -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);