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.
