Messages in this thread Patch in this message | | | From | Hannes Frederic Sowa <> | Subject | [PATCH] random: check for increase of entropy_count because of signed conversion | Date | Wed, 16 Jul 2014 21:18:15 +0200 |
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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.
Reported-by: Dave Jones <davej@redhat.com> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Greg Price <price@mit.edu> Signed-off-by: Hannes Frederic Sowa <hannes@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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