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* [Bug c/98168] New: Optimization that can lead to security vulnerabilities
@ 2020-12-07  4:51 jpegqs at gmail dot com
  2020-12-07  5:10 ` [Bug c/98168] " pinskia at gcc dot gnu.org
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: jpegqs at gmail dot com @ 2020-12-07  4:51 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98168

            Bug ID: 98168
           Summary: Optimization that can lead to security vulnerabilities
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jpegqs at gmail dot com
  Target Milestone: ---

Created attachment 49692
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49692&action=edit
bounds.c

I encountered a bug (98159) that you refused to fix because it is "undefined
behavior". But this code proves that this "compiler" behavior can lead to
security vulnerabilities in some software.

Here GCC thinks that if both signed integers are positive, then the sum of
these integers is also positive. And removes the next bounds check for the
negative values (it could be written different, but this is the common way).

int test(int a, int b, int *buf) {
  if (a >= 0 && b >= 0) {
    a += b;
    // let's check that we are not reading outside the buffer
    if (a >= 0 && a < 8) return buf[a];
  }
  return -1;
}

So this code supposed to read the element A+B from a buffer of 8 values. And if
the sum is out of the buffer, then return -1. But when compiling with GCC
-O2/O3 on x86/x86_64 (and possibly others), you can pass A=0x7fffffff,
B=0x7fffffff and access buf[-2] (as with any negative value except -1).

Thus, optimizations that falsely assume that the target machine is performing
signed integer saturation when it is not - should be considered dangerous.

In my opinion, UB in C has a different purpose, it exists because C is a
low-level language and in most cases can use a single machine instruction for a
general operation. So for compilers it should be "target machine behavior", not
"we can do anything". And compilers must maintain this behavior while removing
some operations when optimizing the code.

^ permalink raw reply	[flat|nested] 6+ messages in thread
end of thread, other threads:[~2020-12-08 10:22 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-12-07  4:51 [Bug c/98168] New: Optimization that can lead to security vulnerabilities jpegqs at gmail dot com
2020-12-07  5:10 ` [Bug c/98168] " pinskia at gcc dot gnu.org
2020-12-07 16:02 ` msebor at gcc dot gnu.org
2020-12-07 21:49 ` egallager at gcc dot gnu.org
2020-12-07 21:57 ` egallager at gcc dot gnu.org
2020-12-08 10:22 ` redi at gcc dot gnu.org

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