If ptr was allocated with an alignment returned from aligned_alloc() and realloc() reallocates memory with a different alignment, the behavior is undefined. This aligned_alloc()function was introduced in the C11 standard.
Noncompliant Code Example
This noncompliant code example aligns ptr to a 4096-byte boundary, whereas the realloc() function aligns the memory to a suitable, but likely different, alignment:
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <stdlib.h>
void func(void) {
size_t resize = 1024;
size_t alignment = 1 << 12;
int *ptr;
int *ptr1;
if ((ptr = aligned_alloc(alignment , sizeof(int))) == NULL) {
/* Handle error */
}
if ((ptr1 = realloc(ptr, resize)) == NULL) {
/* Handle error */
}
} |
The resulting program has undefined behavior when the alignment that realloc() enforces is different from that of aligned_alloc().
Implementation Details
When compiled with GCC 4.1.2 and run on the x86_64 Red Hat Linux platform, the following code produces the following output:
CODE
| Code Block |
|---|
#include <stdlib.h>
#include <stdio.h>
int main(void) {
size_t size = 16;
size_t resize = 1024;
size_t align = 1 << 12;
int *ptr;
int *ptr1;
if (posix_memalign((void **)&ptr, align , size) != 0) {
exit(EXIT_FAILURE);
}
printf("memory aligned to %d bytes\n", align);
printf("ptr = %p\n\n", ptr);
if ((ptr1 = realloc((int *)ptr, resize)) == NULL) {
exit(EXIT_FAILURE);
}
puts("After realloc(): \n");
printf("ptr1 = %p\n", ptr1);
free(ptr1);
return 0;
}
|
OUTPUT
| Code Block |
|---|
memory aligned to 4096 bytes ptr = 0x1621b000 After realloc(): ptr1 = 0x1621a010 |
ptr1 is no longer aligned to 4096 bytes.
Compliant Solution
This compliant solution implements an aligned realloc() function. It allocates resize bytes of new memory with the same alignment as the old memory and then moves the old memory there, then frees the old memory:
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <stdlib.h>
void func(void) {
size_t resize = 1024;
size_t alignment = 1 << 12;
int *ptr;
int *ptr1;
if ((ptr = aligned_alloc(alignment, sizeof(int))) == NULL) {
/* Handle error */
}
if ((ptr1 = aligned_alloc(alignment, resize)) == NULL) {
/* Handle error */
}
if ((memcpy(ptr1, ptr, sizeof(int)) == NULL) {
/* Handle error */
}
free(ptr);
} |
Compliant Solution (Windows)
On Windows, aligned allocations happen via calls to _aligned_malloc() and can be reallocated with calls to _aligned_realloc()[MSDN]. This compliant solution demonstrates one such usage:
| Code Block | ||||
|---|---|---|---|---|
| ||||
#include <malloc.h>
void func(void) {
size_t alignment = 1 << 12;
int *ptr;
int *ptr1;
/* Original allocation */
if ((ptr = _aligned_malloc(sizeof(int), alignment))
== NULL) {
/* Handle error */
}
/* Reallocation */
if ((ptr1 = _aligned_realloc(ptr, 1024, alignment))
== NULL) {
_aligned_free(ptr);
/* Handle error */
}
_aligned_free(ptr1);
} |
Note that on Windows, _aligned_malloc() takes the size and alignment arguments in reverse order from C's aligned_alloc().
Risk Assessment
Improper alignment can lead to arbitrary memory locations being accessed and written to.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
MEM36-C | Low | Probable | High | P2 | L3 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Bibliography
| [ISO/IEC 9899:2011] | Subclause 7.22.3.1, "The aligned_alloc Function" |