random: make 'add_interrupt_randomness()' do something sane

We've been moving away from add_interrupt_randomness() for various
reasons: it's too expensive to do on every interrupt, and flooding the
CPU with interrupts could theoretically cause bogus floods of entropy
from a somewhat externally controllable source.

This solves both problems by limiting the actual randomness addition
to just once a second or after 64 interrupts, whicever comes first.
During that time, the interrupt cycle data is buffered up in a per-cpu
pool.  Also, we make sure the the nonblocking pool used by urandom is
initialized before we start feeding the normal input pool.  This
assures that /dev/urandom is returning unpredictable data as soon as
possible.

(Based on an original patch by Linus, but significantly modified by
tytso.)

Tested-by: Eric Wustrow <ewust@umich.edu>
Reported-by: Eric Wustrow <ewust@umich.edu>
Reported-by: Nadia Heninger <nadiah@cs.ucsd.edu>
Reported-by: Zakir Durumeric <zakir@umich.edu>
Reported-by: J. Alex Halderman <jhalderm@umich.edu>.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org

1 files changed, 86 insertions, 17 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index cb541b9a5231..9fcceace239c 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -127,19 +127,15 @@
  *
  *      void add_input_randomness(unsigned int type, unsigned int code,
  *                                unsigned int value);
- *      void add_interrupt_randomness(int irq);
+ *      void add_interrupt_randomness(int irq, int irq_flags);
  *      void add_disk_randomness(struct gendisk *disk);
  *
  * add_input_randomness() uses the input layer interrupt timing, as well as
  * the event type information from the hardware.
  *
- * add_interrupt_randomness() uses the inter-interrupt timing as random
- * inputs to the entropy pool.  Note that not all interrupts are good
- * sources of randomness!  For example, the timer interrupts is not a
- * good choice, because the periodicity of the interrupts is too
- * regular, and hence predictable to an attacker.  Network Interface
- * Controller interrupts are a better measure, since the timing of the
- * NIC interrupts are more unpredictable.
+ * add_interrupt_randomness() uses the interrupt timing as random
+ * inputs to the entropy pool. Using the cycle counters and the irq source
+ * as inputs, it feeds the randomness roughly once a second.
  *
  * add_disk_randomness() uses what amounts to the seek time of block
  * layer request events, on a per-disk_devt basis, as input to the
@@ -248,6 +244,7 @@
 #include <linux/percpu.h>
 #include <linux/cryptohash.h>
 #include <linux/fips.h>
+#include <linux/ptrace.h>
 
 #ifdef CONFIG_GENERIC_HARDIRQS
 # include <linux/irq.h>
@@ -256,6 +253,7 @@
 #include <asm/processor.h>
 #include <asm/uaccess.h>
 #include <asm/irq.h>
+#include <asm/irq_regs.h>
 #include <asm/io.h>
 
 /*
@@ -421,7 +419,9 @@ struct entropy_store {
         spinlock_t lock;
         unsigned add_ptr;
         int entropy_count;
+        int entropy_total;
         int input_rotate;
+        unsigned int initialized:1;
         __u8 last_data[EXTRACT_SIZE];
 };
 
@@ -454,6 +454,10 @@ static struct entropy_store nonblocking_pool = {
         .pool = nonblocking_pool_data
 };
 
+static __u32 const twist_table[8] = {
+        0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
+        0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
+
 /*
  * This function adds bytes into the entropy "pool".  It does not
  * update the entropy estimate.  The caller should call
@@ -467,9 +471,6 @@ static struct entropy_store nonblocking_pool = {
 static void mix_pool_bytes_extract(struct entropy_store *r, const void *in,
                                    int nbytes, __u8 out[64])
 {
-        static __u32 const twist_table[8] = {
-                0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
-                0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
         unsigned long i, j, tap1, tap2, tap3, tap4, tap5;
         int input_rotate;
         int wordmask = r->poolinfo->poolwords - 1;
@@ -528,6 +529,36 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes)
        mix_pool_bytes_extract(r, in, bytes, NULL);
 }
 
+struct fast_pool {
+        __u32           pool[4];
+        unsigned long   last;
+        unsigned short  count;
+        unsigned char   rotate;
+        unsigned char   last_timer_intr;
+};
+
+/*
+ * This is a fast mixing routine used by the interrupt randomness
+ * collector.  It's hardcoded for an 128 bit pool and assumes that any
+ * locks that might be needed are taken by the caller.
+ */
+static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
+{
+        const char      *bytes = in;
+        __u32           w;
+        unsigned        i = f->count;
+        unsigned        input_rotate = f->rotate;
+
+        while (nbytes--) {
+                w = rol32(*bytes++, input_rotate & 31) ^ f->pool[i & 3] ^
+                        f->pool[(i + 1) & 3];
+                f->pool[i & 3] = (w >> 3) ^ twist_table[w & 7];
+                input_rotate += (i++ & 3) ? 7 : 14;
+        }
+        f->count = i;
+        f->rotate = input_rotate;
+}
+
 /*
  * Credit (or debit) the entropy store with n bits of entropy
  */
@@ -551,6 +582,12 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
                 entropy_count = r->poolinfo->POOLBITS;
         r->entropy_count = entropy_count;
 
+        if (!r->initialized && nbits > 0) {
+                r->entropy_total += nbits;
+                if (r->entropy_total > 128)
+                        r->initialized = 1;
+        }
+
         /* should we wake readers? */
         if (r == &input_pool && entropy_count >= random_read_wakeup_thresh) {
                 wake_up_interruptible(&random_read_wait);
@@ -700,17 +737,48 @@ void add_input_randomness(unsigned int type, unsigned int code,
 }
 EXPORT_SYMBOL_GPL(add_input_randomness);
 
-void add_interrupt_randomness(int irq)
+static DEFINE_PER_CPU(struct fast_pool, irq_randomness);
+
+void add_interrupt_randomness(int irq, int irq_flags)
 {
-        struct timer_rand_state *state;
+        struct entropy_store    *r;
+        struct fast_pool        *fast_pool = &__get_cpu_var(irq_randomness);
+        struct pt_regs          *regs = get_irq_regs();
+        unsigned long           now = jiffies;
+        __u32                   input[4], cycles = get_cycles();
+
+        input[0] = cycles ^ jiffies;
+        input[1] = irq;
+        if (regs) {
+                __u64 ip = instruction_pointer(regs);
+                input[2] = ip;
+                input[3] = ip >> 32;
+        }
 
-        state = get_timer_rand_state(irq);
+        fast_mix(fast_pool, input, sizeof(input));
 
-        if (state == NULL)
+        if ((fast_pool->count & 1023) &&
+            !time_after(now, fast_pool->last + HZ))
                 return;
 
-        DEBUG_ENT("irq event %d\n", irq);
-        add_timer_randomness(state, 0x100 + irq);
+        fast_pool->last = now;
+
+        r = nonblocking_pool.initialized ? &input_pool : &nonblocking_pool;
+        mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));
+        /*
+         * If we don't have a valid cycle counter, and we see
+         * back-to-back timer interrupts, then skip giving credit for
+         * any entropy.
+         */
+        if (cycles == 0) {
+                if (irq_flags & __IRQF_TIMER) {
+                        if (fast_pool->last_timer_intr)
+                                return;
+                        fast_pool->last_timer_intr = 1;
+                } else
+                        fast_pool->last_timer_intr = 0;
+        }
+        credit_entropy_bits(r, 1);
 }
 
 #ifdef CONFIG_BLOCK
@@ -971,6 +1039,7 @@ static void init_std_data(struct entropy_store *r)
 
         spin_lock_irqsave(&r->lock, flags);
         r->entropy_count = 0;
+        r->entropy_total = 0;
         spin_unlock_irqrestore(&r->lock, flags);
 
         now = ktime_get_real();