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CVE PROGRAM SOURCE RECORD7.0 High

CVE-2024-26960

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

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Status
Published
Published
Updated
Assigning organization
Linux

Description

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<-----

Affected products

Version statuses are supplied by the assigning organization. Consult the original record for detailed ranges and platform conditions.

Linux Linux

Default version status: unaffected

  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < d85c11c97ecf92d47a4b29e3faca714dc1f18d0daffected
  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < 2da5568ee222ce0541bfe446a07998f92ed1643eaffected
  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < 1ede7f1d7eed1738d1b9333fd1e152ccb450b86aaffected
  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < 0f98f6d2fb5fad00f8299b84b85b6bc1b6d7d19aaffected
  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < 3ce4c4c653e4e478ecb15d3c88e690f12cbf6b39affected
  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < 363d17e7f7907c8e27a9e86968af0eaa2301787baffected
  • 7c00bafee87c7bac7ed9eced7c161f8e5332cb4e to < 82b1c07a0af603e3c47b906c8e991dc96f01688eaffected

Linux Linux

Default version status: affected

  • 4.11affected
  • 0 to < 4.11unaffected
  • 5.10.215 to ≤ 5.10.*unaffected
  • 5.15.154 to ≤ 5.15.*unaffected
  • 6.1.84 to ≤ 6.1.*unaffected
  • 6.6.24 to ≤ 6.6.*unaffected
  • 6.7.12 to ≤ 6.7.*unaffected
  • 6.8.3 to ≤ 6.8.*unaffected
  • 6.9 to ≤ *unaffected

References