random: fix BUG_ON caused by accounting simplification

Commit d7e58560111160f5478e13b2387d6b20c4102937 ("random: simplify accounting logic") simplified
things too much, in that it allows the following to trigger an
overflow that results in a BUG_ON crash:

dd if=/dev/urandom of=/dev/zero bs=67108707 count=1

Thanks to Peter Zihlstra for discovering the crash, and Hannes
Frederic for analyzing the root cause.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Greg Price <price@mit.edu>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
random: convert use of typedef ctl_table to struct ctl_table

This typedef is unnecessary and should just be removed.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
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: "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
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
random: check for increase of entropy_count because of signed conversion

The expression entropy_count -= ibytes << (ENTROPY_SHIFT + 3) could
actually increase entropy_count if during assignment of the unsigned
expression on the RHS (mind the -=) we reduce the value modulo
2^width(int) and assign it to entropy_count. Trinity found this.

[ Commit modified by tytso to add an additional safety check for a
  negative entropy_count -- which should never happen, and to also add
  an additional paranoia check to prevent overly large count values to
  be passed into urandom_read().  ]

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>