[PATCH] random: check for increase of entropy_count because of signed conversion

[Hannes Frederic Sowa]

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 <davej@xxxxxxxxxx>
Cc: Theodore Ts'o <tytso@xxxxxxx>
Cc: Greg Price <price@xxxxxxx>
Signed-off-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
---
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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