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random: check for increase of entropy_count because of signed conversion

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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
This commit is contained in:
Hannes Frederic Sowa 2014-07-18 17:26:41 -04:00 committed by Theodore Ts'o
parent 1795cd9b3a
commit 79a8468747
1 changed files with 14 additions and 3 deletions
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17
drivers/char/random.c
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@ -641,7 +641,7 @@ retry:
} while (unlikely(entropy_count < pool_size-2 && pnfrac));
}
if (entropy_count < 0) {
if (unlikely(entropy_count < 0)) {
pr_warn("random: negative entropy/overflow: pool %s count %d\n",
r->name, entropy_count);
WARN_ON(1);
@ -981,7 +981,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
int reserved)
{
int entropy_count, orig;
size_t ibytes;
size_t ibytes, nfrac;
BUG_ON(r->entropy_count > r->poolinfo->poolfracbits);
@ -999,7 +999,17 @@ retry:
}
if (ibytes < min)
ibytes = 0;
if ((entropy_count -= ibytes << (ENTROPY_SHIFT + 3)) < 0)
if (unlikely(entropy_count < 0)) {
pr_warn("random: negative entropy count: pool %s count %d\n",
r->name, entropy_count);
WARN_ON(1);
entropy_count = 0;
}
nfrac = ibytes << (ENTROPY_SHIFT + 3);
if ((size_t) entropy_count > nfrac)
entropy_count -= nfrac;
else
entropy_count = 0;
if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
@ -1376,6 +1386,7 @@ urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
"with %d bits of entropy available\n",
current->comm, nonblocking_pool.entropy_total);
nbytes = min_t(size_t, nbytes, INT_MAX >> (ENTROPY_SHIFT + 3));
ret = extract_entropy_user(&nonblocking_pool, buf, nbytes);
trace_urandom_read(8 * nbytes, ENTROPY_BITS(&nonblocking_pool),