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author | Theodore Ts'o <tytso@mit.edu> | 2014-06-15 21:04:32 -0400 |
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committer | Theodore Ts'o <tytso@mit.edu> | 2014-06-15 21:04:32 -0400 |
commit | e33ba5fa7afce1a9f159704121d4e4d110df8185 (patch) | |
tree | ec1d1728fea1e7d20351cb99ae992356a98e7e12 /drivers/char/random.c | |
parent | 1860e379875dfe7271c649058aeddffe5afd9d0d (diff) | |
download | linux-e33ba5fa7afce1a9f159704121d4e4d110df8185.tar.bz2 |
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
Diffstat (limited to 'drivers/char/random.c')
-rw-r--r-- | drivers/char/random.c | 17 |
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; |