CVE-2024-26960 mm: swap: fix race between free_swap_and_cache() and swapoff()

2024-05-0105:19:12

Linux

github.com

linux kernel

mm swap

free swap and cache

swapoff

race condition

vulnerability fix

6.8 Medium

AI Score

Confidence

Low

0.0004 Low

EPSS

Percentile

10.4%

JSON

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

mm: swap: fix race between free_swap_and_cache() and swapoff()

There was previously a theoretical window where swapoff() could run and
teardown a swap_info_struct while a call to free_swap_and_cache() was
running in another thread. This could cause, amongst other bad
possibilities, swap_page_trans_huge_swapped() (called by
free_swap_and_cache()) to access the freed memory for swap_map.

This is a theoretical problem and I haven’t been able to provoke it from a
test case. But there has been agreement based on code review that this is
possible (see link below).

Fix it by using get_swap_device()/put_swap_device(), which will stall
swapoff(). There was an extra check in _swap_info_get() to confirm that
the swap entry was not free. This isn’t present in get_swap_device()
because it doesn’t make sense in general due to the race between getting
the reference and swapoff. So I’ve added an equivalent check directly in
free_swap_and_cache().

Details of how to provoke one possible issue (thanks to David Hildenbrand
for deriving this):

–8<-----

__swap_entry_free() might be the last user and result in
“count == SWAP_HAS_CACHE”.

swapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.

So the question is: could someone reclaim the folio and turn
si->inuse_pages==0, before we completed swap_page_trans_huge_swapped().

Imagine the following: 2 MiB folio in the swapcache. Only 2 subpages are
still references by swap entries.

Process 1 still references subpage 0 via swap entry.
Process 2 still references subpage 1 via swap entry.

Process 1 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE
[then, preempted in the hypervisor etc.]

Process 2 quits. Calls free_swap_and_cache().
-> count == SWAP_HAS_CACHE

Process 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls
__try_to_reclaim_swap().

__try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->
put_swap_folio()->free_swap_slot()->swapcache_free_entries()->
swap_entry_free()->swap_range_free()->
…
WRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);

What stops swapoff to succeed after process 2 reclaimed the swap cache
but before process1 finished its call to swap_page_trans_huge_swapped()?

–8<-----

CNA Affected

[
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "d85c11c97ecf",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "2da5568ee222",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "1ede7f1d7eed",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "0f98f6d2fb5f",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "3ce4c4c653e4",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "363d17e7f790",
        "versionType": "git"
      },
      {
        "status": "affected",
        "version": "7c00bafee87c",
        "lessThan": "82b1c07a0af6",
        "versionType": "git"
      }
    ],
    "programFiles": [
      "mm/swapfile.c"
    ],
    "defaultStatus": "unaffected"
  },
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "4.11"
      },
      {
        "status": "unaffected",
        "version": "0",
        "lessThan": "4.11",
        "versionType": "custom"
      },
      {
        "status": "unaffected",
        "version": "5.10.215",
        "versionType": "custom",
        "lessThanOrEqual": "5.10.*"
      },
      {
        "status": "unaffected",
        "version": "5.15.154",
        "versionType": "custom",
        "lessThanOrEqual": "5.15.*"
      },
      {
        "status": "unaffected",
        "version": "6.1.84",
        "versionType": "custom",
        "lessThanOrEqual": "6.1.*"
      },
      {
        "status": "unaffected",
        "version": "6.6.24",
        "versionType": "custom",
        "lessThanOrEqual": "6.6.*"
      },
      {
        "status": "unaffected",
        "version": "6.7.12",
        "versionType": "custom",
        "lessThanOrEqual": "6.7.*"
      },
      {
        "status": "unaffected",
        "version": "6.8.3",
        "versionType": "custom",
        "lessThanOrEqual": "6.8.*"
      },
      {
        "status": "unaffected",
        "version": "6.9",
        "versionType": "original_commit_for_fix",
        "lessThanOrEqual": "*"
      }
    ],
    "programFiles": [
      "mm/swapfile.c"
    ],
    "defaultStatus": "affected"
  }
]