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Date: Tue, 25 Jun 2019 09:37:53 -0600
From: Jeff Law <law@...hat.com>
To: Pascal Cuoq <cuoq@...st-in-soft.com>,
 "oss-security@...ts.openwall.com" <oss-security@...ts.openwall.com>
Subject: Re: Thousands of vulnerabilities, almost no CVEs:
 OSS-Fuzz

On 6/25/19 9:02 AM, Pascal Cuoq wrote:
> Hello,
> 
>> On 25 Jun 2019, at 16:33, Jeff Law <law@...hat.com> wrote:
>> 
>> On 6/25/19 8:14 AM, Matthew Fernandez wrote:
>>> 
>>> 
>>> 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.
> 
> This is tangential to the subject and perhaps we should take this
> sub-discussion off the list, or at least make a new thread. I'm
> interested in your opinion of what is going on with Ubuntu's packaged
> GCC version 4.4.3 in the example under the section “The next example”
> in this blog post, where this very thing is happening (when invoked
> with fewer than 3 arguments, the compiled code claims that the result
> of an unsigned multiplication by 2 is odd):
> 
> http://blog.frama-c.com/index.php?post/2013/03/13/indeterminate-undefined
>
>  I have not been able to reproduce this with any of the GCC versions
> hosted at Compiler Explorer, so I believe that this may never have
> been part of the GCC official tree, but the fact remains that all of
> Ubuntu 4.4.3 and all source programs that were compiled with Ubuntu
> 4.4.3's default compiler were compiled with a compiler that did this
> (surprising, dangerous in some contexts) thing.


#include <stdio.h>

int main(int c, char **v)
{
  unsigned int j;
  if (c==4)
    j = 1;
  else
    j *= 2;
  printf("j:%u ",j);
  printf("c:%d\n",c);
}

Just like the first example, the compiler can choose any value for the
input and output of the j *= 2 statement.  It does not have to be even.
 The compiler does not have to actually perform the multiplication or
any computation for that matter.  It can just use whatever value is
conveniently lying around in a register for the result.

In theory that would in turn allow the compiler to elide the conditional
because it could make the else clause be "j = 1" and once the then/else
are identical the conditional can be eliminated because it serves no
purpose.  GCC doesn't do that, but it could.  I believe LLVM is more
aggressive in this kind of stuff.

Now if the else clause has an observable side effect, then that would
prevent this kind of optimization because those side effects still have
to happen.

Jeff


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