Re: [RFC] tcp: randomize TCP source ports

Hannes Frederic Sowa
2013-11-09T04:47:26Z

On Fri, Nov 08, 2013 at 07:11:18AM -0800, Eric Dumazet wrote:
> On Fri, 2013-11-08 at 15:28 +0100, Hannes Frederic Sowa wrote:
>
> > What do you think about using a timer to keep the reseed out of fast-path
> > and switch to the non-arch get_random_bytes instead?
>
> Well, the initial seed value is get_random_bytes(). I felt that using a
> xor with the _arch() version would be safe enough.
>
> For the timer, I do not think its worth the pain : Do you want a per cpu
> timer, or a global one ?

This untested diff came to my mind (it is based on the random tree). I
actually consider to propose something like this for 3.13. UDP port
randomization is really critical.

In 3.14 timeframe I suggest abandon net_random and use prandom_u32
directly so code gets easier to audit.

Would it hurt to use "proper" get_random_byte calls for port randomization?

diff --git a/drivers/char/random.c b/drivers/char/random.c
index cdf4cfb..e9d0136 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -657,9 +657,11 @@ retry:
r->entropy_total += nbits;
if (!r->initialized && nbits > 0) {
if (r->entropy_total > 128) {
- if (r == &nonblocking_pool)
+ if (r == &nonblocking_pool) {
pr_notice("random: %s pool is initialized\n",
r->name);
+ prandom_reseed();
+ }
r->initialized = 1;
r->entropy_total = 0;
}
diff --git a/include/linux/random.h b/include/linux/random.h
index 6312dd9..4f878c0 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -29,6 +29,7 @@ unsigned long randomize_range(unsigned long start, unsigned long end, unsigned l
u32 prandom_u32(void);
void prandom_bytes(void *buf, int nbytes);
void prandom_seed(u32 seed);
+void prandom_reseed(void);

u32 prandom_u32_state(struct rnd_state *);
void prandom_bytes_state(struct rnd_state *state, void *buf, int nbytes);
diff --git a/lib/random32.c b/lib/random32.c
index 52280d5..1ee611f 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -174,11 +174,31 @@ static int __init prandom_init(void)
}
core_initcall(prandom_init);

+static void __prandom_timer(unsigned long dontcare);
+static DEFINE_TIMER(seed_timer, __prandom_timer, 0, 0);
+
+static void __prandom_timer(unsigned long dontcare)
+{
+ u32 entropy;
+ get_random_bytes(&entropy, sizeof(entropy));
+ prandom_seed(entropy);
+ seed_timer.expires = jiffies + 60 * HZ;
+ add_timer(&seed_timer);
+}
+
+static int prandom_start_seed_timer(void)
+{
+ seed_timer.expires = jiffies + 60 * HZ;
+ add_timer(&seed_timer);
+ return 0;
+}
+late_initcall(prandom_start_seed_timer);
+
/*
* Generate better values after random number generator
* is fully initialized.
*/
-static int __init prandom_reseed(void)
+void prandom_reseed(void)
{
int i;

@@ -196,4 +216,3 @@ static int __init prandom_reseed(void)
}
return 0;
}
-late_initcall(prandom_reseed);

Greetings,

Hannes