Recently, GCC began to assume for optimization purposes that p and q are
non-null pointers when
memcpy(p, q, n); is invoked.
This means that the if is eliminated completely when compiling the following
sequence of instructions:
memcpy(p, q, n);
if (!p) printf("good\n");
And this causes a problem for any programmer that would expect “good” to be
printed by the
following program:
#include <string.h>
void f(void *p, void *q, size_t n) {
memcpy(p, q, n);
if (!p) printf("good\n");
}
int main(void) {
f(0, 0, 0);
}
The clauses in the standard that allow GCC to “optimize” the program in this
way are, in C11, 7.24.1:2 and 7.1.4.
Clause 7.24.1:2 says:
“Where an argument declared as size_t n specifies the length of the array for a
function,
n can have the value zero on a call to that function. Unless explicitly stated
otherwise in the description
of a particular function in this subclause, pointer arguments on such a call
shall still have valid values,
as described in 7.1.4”
Clause 7.1.4 also allows compilers to assume that p and q are not pointers “one
past” the end of an object:
http://stackoverflow.com/questions/25390577/is-memcpya-1-b-1-0-defined-in-c11
Since you can expect GCC developers to take as much responsibility for the
vulnerabilities
introduced in previously working code when they add the optimization of
assuming that p and q
are not pointers “one past” than they did when they added the optimization of
assuming that p
and q are not null (i.e. none at all), it would be prudent never to call any
standard function
with pointers “one past”, even when these are functions that also take a length
and the length
is always 0 in these cases.
OpenSSL's bignum implementation contains two invocations of standard functions
that
fail this property:
https://github.com/openssl/openssl/blob/b39fc560612984e65ec30d7f37487303bf514fb3/crypto/bn/bn_add.c#L225
https://github.com/openssl/openssl/blob/b39fc560612984e65ec30d7f37487303bf514fb3/crypto/bn/bn_mont.c#L199
These two lines are actually reached with pointers “one past” and sizes of 0
during real executions.
The prudent thing to do would be to guard these lines so that the standard
function is not called
with a pointer “one past”, which can be done as simply as:
if (max - r->top) memset(&rp[r->top], 0, sizeof(*rp) * (max - r->top));
if (dif) memcpy(rp, ap, sizeof(*rp) * dif);
_______________________________________________
openssl-bugs-mod mailing list
openssl-bugs-...@openssl.org
https://mta.openssl.org/mailman/listinfo/openssl-bugs-mod
_______________________________________________
openssl-dev mailing list
To unsubscribe: https://mta.openssl.org/mailman/listinfo/openssl-dev
