From: Theodore Ts'o <tytso@mit.edu>
Date: Mon, 16 Jun 2014 01:04:32 +0000 (-0400)
Subject: random: fix nasty entropy accounting bug
X-Git-Tag: omap-for-v3.16/fixes-rc4~88^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e33ba5fa7afce1a9f159704121d4e4d110df8185;p=pandora-kernel.git

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
---

Reading git-diff-tree failed