random: fix nasty entropy accounting bug

Commit 0fb7a01af5b0 "random: simplify accounting code", introduced in
v3.15, has a very nasty accounting problem when the entropy pool has
has fewer bytes of entropy than the number of requested reserved
bytes.  In that case, "have_bytes - reserved" goes negative, and since
size_t is unsigned, the expression:

       ibytes = min_t(size_t, ibytes, have_bytes - reserved);

... does not do the right thing.  This is rather bad, because it
defeats the catastrophic reseeding feature in the
xfer_secondary_pool() path.

It also can cause the "BUG: spinlock trylock failure on UP" for some
kernel configurations when prandom_reseed() calls get_random_bytes()
in the early init, since when the entropy count gets corrupted,
credit_entropy_bits() erroneously believes that the nonblocking pool
has been fully initialized (when in fact it is not), and so it calls
prandom_reseed(true) recursively leading to the spinlock BUG.

The logic is *not* the same it was originally, but in the cases where
it matters, the behavior is the same, and the resulting code is
hopefully easier to read and understand.

Fixes: 0fb7a01af5b0 "random: simplify accounting code"
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Greg Price <price@mit.edu>
Cc: stable@vger.kernel.org  #v3.15

1 files changed, 9 insertions, 8 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 102c50d38902..2b6e4cd8de8e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -979,7 +979,6 @@ static void push_to_pool(struct work_struct *work)
 static size_t account(struct entropy_store *r, size_t nbytes, int min,
                       int reserved)
 {
-        int have_bytes;
         int entropy_count, orig;
         size_t ibytes;
 
@@ -988,17 +987,19 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
         /* Can we pull enough? */
 retry:
         entropy_count = orig = ACCESS_ONCE(r->entropy_count);
-        have_bytes = entropy_count >> (ENTROPY_SHIFT + 3);
         ibytes = nbytes;
         /* If limited, never pull more than available */
-        if (r->limit)
-                ibytes = min_t(size_t, ibytes, have_bytes - reserved);
+        if (r->limit) {
+                int have_bytes = entropy_count >> (ENTROPY_SHIFT + 3);
+
+                if ((have_bytes -= reserved) < 0)
+                        have_bytes = 0;
+                ibytes = min_t(size_t, ibytes, have_bytes);
+        }
         if (ibytes < min)
                 ibytes = 0;
-        if (have_bytes >= ibytes + reserved)
-                entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
-        else
-                entropy_count = reserved << (ENTROPY_SHIFT + 3);
+        if ((entropy_count -= ibytes << (ENTROPY_SHIFT + 3)) < 0)
+                entropy_count = 0;
 
         if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
                 goto retry;