CVE-2024-53111

In the Linux kernel, the following vulnerability has been resolved:

mm/mremap: fix address wraparound in movepagetables()

On 32-bit platforms, it is possible for the expression len + old_addr < old_end to be false-positive if len + old_addr wraps around. old_addr is the cursor in the old range up to which page table entries have been moved; so if the operation succeeded, old_addr is the end of the old region, and adding len to it can wrap.

The overflow causes mremap() to mistakenly believe that PTEs have been copied; the consequence is that mremap() bails out, but doesn't move the PTEs back before the new VMA is unmapped, causing anonymous pages in the region to be lost. So basically if userspace tries to mremap() a private-anon region and hits this bug, mremap() will return an error and the private-anon region's contents appear to have been zeroed.

The idea of this check is that old_end - len is the original start address, and writing the check that way also makes it easier to read; so fix the check by rearranging the comparison accordingly.

(An alternate fix would be to refactor this function by introducing an "origoldstart" variable or such.)

Tested in a VM with a 32-bit X86 kernel; without the patch:

user@horn:~/big_mremap$ cat test.c
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <err.h>
#include <sys/mman.h>

#define ADDR1 ((void*)0x60000000)
#define ADDR2 ((void*)0x10000000)
#define SIZE          0x50000000uL

int main(void) {
  unsigned char *p1 = mmap(ADDR1, SIZE, PROT_READ|PROT_WRITE,
      MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED_NOREPLACE, -1, 0);
  if (p1 == MAP_FAILED)
    err(1, "mmap 1");
  unsigned char *p2 = mmap(ADDR2, SIZE, PROT_NONE,
      MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED_NOREPLACE, -1, 0);
  if (p2 == MAP_FAILED)
    err(1, "mmap 2");
  *p1 = 0x41;
  printf("first char is 0x%02hhx\n", *p1);
  unsigned char *p3 = mremap(p1, SIZE, SIZE,
      MREMAP_MAYMOVE|MREMAP_FIXED, p2);
  if (p3 == MAP_FAILED) {
    printf("mremap() failed; first char is 0x%02hhx\n", *p1);
  } else {
    printf("mremap() succeeded; first char is 0x%02hhx\n", *p3);
  }
}
user@horn:~/big_mremap$ gcc -static -o test test.c
user@horn:~/big_mremap$ setarch -R ./test
first char is 0x41
mremap() failed; first char is 0x00

With the patch:

user@horn:~/big_mremap$ setarch -R ./test
first char is 0x41
mremap() succeeded; first char is 0x41