random: check for increase of entropy_count because of signed conversion
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 <d5ec0d24602845f6f4b6a35ef729998d1fba5507@redhat.com>
Signed-off-by: Hannes Frederic Sowa <331be22c6b63ca3e0a03d408c2d906b1b02cd5f2@stressinduktion.org>
Signed-off-by: Theodore Ts'o <4ed386e0495d3e109932df055831d9ec2f824927@mit.edu>
Cc: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs