On Mon, Jan 16, 2012 at 01:19:22PM -0500, Scott Lovenberg wrote:
> Let me walk you guys through how this bug could be exploited.
> The file that you want to access is blocked from you by file system
> permissions. The root user (uid==0) can access this file (that contains
> credentials) and read it into memory that it has malloc()'ed. After the
> process running as root is done, it free()'s the memory without zeroing it
> out. Now you (you clever hacker) spawn a process that requests memory in
> large hunks. It then searches for the string "password=" in that memory.
> Since the memory was free()'ed back to the pool without being changed, it
> still contains the original information that was in the file that you
> cannot read. Does this make sense, or should I go into t a bit more detail?
But can you actually get this dirty memory on Linux?
I know two sources of memory that are used by malloc. One is brk(), the
other is mmapped pages of /dev/zero. With /dev/zero it's obvious that
you get empty pages (all-zero); with brk I wasn't sure so I wrote the
test program below and ran it. I didn't find any dirty (non-zero) memory.
Thanks,
Jonathan Neuschäfer
--
#include <unistd.h>
#include <stdio.h>
#define BLOCKSZ (1024 * 1024) /* one Mibi */
int main(void)
{
int maxmb = 1024;
unsigned i;
void *BRK;
BRK = sbrk(0);
for (i = 0; i < maxmb; i++) {
void *block = sbrk(BLOCKSZ);
unsigned j, *p;
if (block == (void *) -1) {
printf("sbrk failed after %u blocks (%u bytes)\n", i, i * BLOCKSZ);
break;
}
for (p = block, j = BLOCKSZ/sizeof(unsigned int); j--; p++)
if (*p)
printf("found data at BRK+%p: %u\n", ((void *)p) - BRK, *p);
}
return 0;
}
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