Does Linux process exist information leakage?

[Jonathan Neuschäfer]

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;
}