CVE-2024-26960 (GCVE-0-2024-26960)
Vulnerability from cvelistv5
Published
2024-05-01 05:19
Modified
2026-05-12 11:50
Severity ?
5.5 (Medium) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Summary
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<-----
References
Impacted products
Vendor Product Version
Linux Linux Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
Version: 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: swap: fix race between free_swap_and_cache() and swapoff()\n\nThere was previously a theoretical window where swapoff() could run and\nteardown a swap_info_struct while a call to free_swap_and_cache() was\nrunning in another thread.  This could cause, amongst other bad\npossibilities, swap_page_trans_huge_swapped() (called by\nfree_swap_and_cache()) to access the freed memory for swap_map.\n\nThis is a theoretical problem and I haven\u0027t been able to provoke it from a\ntest case.  But there has been agreement based on code review that this is\npossible (see link below).\n\nFix it by using get_swap_device()/put_swap_device(), which will stall\nswapoff().  There was an extra check in _swap_info_get() to confirm that\nthe swap entry was not free.  This isn\u0027t present in get_swap_device()\nbecause it doesn\u0027t make sense in general due to the race between getting\nthe reference and swapoff.  So I\u0027ve added an equivalent check directly in\nfree_swap_and_cache().\n\nDetails of how to provoke one possible issue (thanks to David Hildenbrand\nfor deriving this):\n\n--8\u003c-----\n\n__swap_entry_free() might be the last user and result in\n\"count == SWAP_HAS_CACHE\".\n\nswapoff-\u003etry_to_unuse() will stop as soon as soon as si-\u003einuse_pages==0.\n\nSo the question is: could someone reclaim the folio and turn\nsi-\u003einuse_pages==0, before we completed swap_page_trans_huge_swapped().\n\nImagine the following: 2 MiB folio in the swapcache. Only 2 subpages are\nstill references by swap entries.\n\nProcess 1 still references subpage 0 via swap entry.\nProcess 2 still references subpage 1 via swap entry.\n\nProcess 1 quits. Calls free_swap_and_cache().\n-\u003e count == SWAP_HAS_CACHE\n[then, preempted in the hypervisor etc.]\n\nProcess 2 quits. Calls free_swap_and_cache().\n-\u003e count == SWAP_HAS_CACHE\n\nProcess 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls\n__try_to_reclaim_swap().\n\n__try_to_reclaim_swap()-\u003efolio_free_swap()-\u003edelete_from_swap_cache()-\u003e\nput_swap_folio()-\u003efree_swap_slot()-\u003eswapcache_free_entries()-\u003e\nswap_entry_free()-\u003eswap_range_free()-\u003e\n...\nWRITE_ONCE(si-\u003einuse_pages, si-\u003einuse_pages - nr_entries);\n\nWhat stops swapoff to succeed after process 2 reclaimed the swap cache\nbut before process1 finished its call to swap_page_trans_huge_swapped()?\n\n--8\u003c-----"
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      "title": "mm: swap: fix race between free_swap_and_cache() and swapoff()",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2024-26960",
    "datePublished": "2024-05-01T05:19:12.112Z",
    "dateReserved": "2024-02-19T14:20:24.201Z",
    "dateUpdated": "2026-05-12T11:50:50.097Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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Sightings

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  • Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
  • Confirmed: The vulnerability is confirmed from an analyst perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
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