linux-kernel - Re: [PATCH] crypto: drbg: check blocklen is non zero

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Message-ID: <97371bc7-ebb6-92f2-bbaa-6b444e27cb8b@redhat.com>
Date:   Thu, 20 Aug 2020 06:27:36 -0700
From:   Tom Rix <trix@...hat.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     davem@...emloft.net, smueller@...onox.de,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] crypto: drbg: check blocklen is non zero

There are many divide by 0 reports.  This one got attention because it is in crypto, where i believe problems, even false positives, should be fixed.

Tom

On 8/20/20 12:15 AM, Herbert Xu wrote:
> On Sun, Aug 02, 2020 at 10:12:47AM -0700, trix@...hat.com wrote:
>> From: Tom Rix <trix@...hat.com>
>>
>> Clang static analysis reports this error
>>
>> crypto/drbg.c:441:40: warning: Division by zero
>>         padlen = (inputlen + sizeof(L_N) + 1) % (drbg_blocklen(drbg));
>>                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
>>
>> When drbg_bocklen fails it returns 0.
>>
>> 	if (drbg && drbg->core)
>> 		return drbg->core->blocklen_bytes;
>> 	return 0;
> Yes but it can only fail if the drbg is not instantiated.  If
> you're hitting the generate path with an uninstantiated drbg you've
> got bigger problems than a divide by zero.
>
> So how is this even possible?
>
> Cheers,