[PATCH] swapfile: fix memory corruption via malformed swapfile

[Jann Horn <jann@thejh.net>]

When root activates a swap partition whose header has the wrong endianness,
nr_badpages elements of badpages are swabbed before nr_badpages has been
checked, leading to a buffer overrun of up to 8GB.

This normally is not a security issue because it can only be exploited by
root (more specifically, a process with CAP_SYS_ADMIN or the ability to
modify a swap file/partition), and such a process can already e.g. modify
swapped-out memory of any other userspace process on the system.

Testcase for reproducing the bug (must be run as root, should crash your
kernel):
=================
#include <stdlib.h>
#include <unistd.h>
#include <sys/swap.h>
#include <limits.h>
#include <err.h>
#include <string.h>
#include <stdio.h>

#define PAGE_SIZE 4096
#define __u32 unsigned int


// from include/linux/swap.h
union swap_header {
  struct {
    char reserved[PAGE_SIZE - 10];
    char magic[10];     /* SWAP-SPACE or SWAPSPACE2 */
  } magic;
  struct {
    char    bootbits[1024]; /* Space for disklabel etc. */
    __u32   version;
    __u32   last_page;
    __u32   nr_badpages;
    unsigned char sws_uuid[16];
    unsigned char sws_volume[16];
    __u32   padding[117];
    __u32   badpages[1];
  } info;
};

int main(void) {
  char file[] = "/tmp/swapfile.XXXXXX";
  int file_fd = mkstemp(file);
  if (file_fd == -1)
    err(1, "mkstemp");
  if (ftruncate(file_fd, PAGE_SIZE))
    err(1, "ftruncate");
  union swap_header swap_header = {
    .info = {
      .version = __builtin_bswap32(1),
      .nr_badpages = __builtin_bswap32(INT_MAX)
    }
  };
  memcpy(swap_header.magic.magic, "SWAPSPACE2", 10);
  if (write(file_fd, &swap_header, sizeof(swap_header)) !=
      sizeof(swap_header))
    err(1, "write");

  // not because the attack needs it, just in case you forgot to
  // sync yourself before crashing your machine
  sync();

  // now die
  if (swapon(file, 0))
    err(1, "swapon");
  puts("huh, we survived");
  if (swapoff(file))
    err(1, "swapoff");
  unlink(file);
}
=================

Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jann@thejh.net>
---
 mm/swapfile.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2210de290b54..f30438970cd1 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2224,6 +2224,8 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 		swab32s(&swap_header->info.version);
 		swab32s(&swap_header->info.last_page);
 		swab32s(&swap_header->info.nr_badpages);
+		if (swap_header->info.nr_badpages > MAX_SWAP_BADPAGES)
+			return 0;
 		for (i = 0; i < swap_header->info.nr_badpages; i++)
 			swab32s(&swap_header->info.badpages[i]);
 	}
-- 
2.1.4