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.
Reported-by: Dave Jones <da...@redhat.com>
Cc: Theodore Ts'o <ty...@mit.edu>
Cc: Greg Price <pr...@mit.edu>
Signed-off-by: Hannes Frederic Sowa <han...@stressinduktion.org>
---
As indicated by credit_entropy_bits entropy_count cannot get negative,
so I don't see any reason to include a check for entropy_count < 0
here. Do you agree?
drivers/char/random.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0a7ac0a..cd50c4e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -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 +999,11 @@ retry:
}
if (ibytes < min)
ibytes = 0;
- if ((entropy_count -= ibytes << (ENTROPY_SHIFT + 3)) < 0)
+
+ nfrac = ibytes << (ENTROPY_SHIFT + 3);
+ if (entropy_count > nfrac)
+ entropy_count -= nfrac;
+ else
entropy_count = 0;
if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
--
1.9.3
--
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