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Date: Tue, 25 Jun 2019 08:33:42 -0600
From: Jeff Law <law@...hat.com>
To: oss-security@...ts.openwall.com,
Matthew Fernandez <matthew.fernandez@...il.com>
Subject: Re: Thousands of vulnerabilities, almost no CVEs:
OSS-Fuzz
On 6/25/19 8:14 AM, Matthew Fernandez wrote:
>
>> On Jun 25, 2019, at 06:41, Bob Friesenhahn <bfriesen@...ple.dallas.tx.us> wrote:
>>
>> * Consumption of uninitialized data (e.g. image data) which is not
>> used to make important decisions. This is usually due to unhandled
>> cases or error handling which does not quit immediately.
>
> C/C++ compilers will infer backwards from uninitialized variable reads (undefined behavior in these languages) that preceding code is unreachable. For example, when moving from GCC 6 series to GCC 7 series we found one of our code bases would produce a binary that would only segfault when compiled at >= -O2. We root caused this to exactly the situation you describe: an error handling path that read uninitialized variables. The compiler appeared to infer backwards that the error check itself was a no-op as the true branch led to unconditional UB (this is my interpretation of its actions; I did not delve into the compiler’s internals).
Well, as a GCC developer, I can say it doesn't use an uninitialized read
to allow back-propagation of state to eliminate conditionals. It may
have looked that way, but there had to be something else going on.
Jeff
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