linux-kernel - Coverity: add_early_randomness(): Integer handling issues

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Date:   Tue, 8 Nov 2022 09:29:27 -0800
From:   coverity-bot <keescook@...omium.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Olivia Mackall <olivia@...enic.com>,
        linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-next@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Coverity: add_early_randomness(): Integer handling issues

Hello!

This is an experimental semi-automated report about issues detected by
Coverity from a scan of next-20221108 as part of the linux-next scan project:
https://scan.coverity.com/projects/linux-next-weekly-scan

You're getting this email because you were associated with the identified
lines of code (noted below) that were touched by commits:

  Mon Nov 7 12:47:57 2022 +0100
    e0a37003ff0b ("hw_random: use add_hwgenerator_randomness() for early entropy")

Coverity reported the following:

*** CID 1527234:  Integer handling issues  (SIGN_EXTENSION)
drivers/char/hw_random/core.c:73 in add_early_randomness()
67     	int bytes_read;
68
69     	mutex_lock(&reading_mutex);
70     	bytes_read = rng_get_data(rng, rng_fillbuf, 32, 0);
71     	mutex_unlock(&reading_mutex);
72     	if (bytes_read > 0) {
vvv     CID 1527234:  Integer handling issues  (SIGN_EXTENSION)
vvv     Suspicious implicit sign extension: "rng->quality" with type "unsigned short" (16 bits, unsigned) is promoted in "bytes_read * 8 * rng->quality / 1024" to type "int" (32 bits, signed), then sign-extended to type "unsigned long" (64 bits, unsigned).  If "bytes_read * 8 * rng->quality / 1024" is greater than 0x7FFFFFFF, the upper bits of the result will all be 1.
73     		size_t entropy = bytes_read * 8 * rng->quality / 1024;
74     		add_hwgenerator_randomness(rng_fillbuf, bytes_read, entropy, false);
75     	}
76     }
77
78     static inline void cleanup_rng(struct kref *kref)

If this is a false positive, please let us know so we can mark it as
such, or teach the Coverity rules to be smarter. If not, please make
sure fixes get into linux-next. :) For patches fixing this, please
include these lines (but double-check the "Fixes" first):

Reported-by: coverity-bot <keescook+coverity-bot@...omium.org>
Addresses-Coverity-ID: 1527234 ("Integer handling issues")
Fixes: e0a37003ff0b ("hw_random: use add_hwgenerator_randomness() for early entropy")

Thanks for your attention!

-- 
Coverity-bot