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<-----
History

Fri, 22 Nov 2024 12:00:00 +0000

Type Values Removed Values Added
References

Wed, 13 Nov 2024 02:45:00 +0000

Type Values Removed Values Added
CPEs cpe:/a:redhat:enterprise_linux:9
cpe:/o:redhat:enterprise_linux:9

Tue, 05 Nov 2024 10:45:00 +0000

Type Values Removed Values Added
References

Thu, 08 Aug 2024 19:15:00 +0000

Type Values Removed Values Added
First Time appeared Redhat
Redhat enterprise Linux
CPEs cpe:/a:redhat:enterprise_linux:8::nfv
cpe:/o:redhat:enterprise_linux:8
Vendors & Products Redhat
Redhat enterprise Linux

cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published: 2024-05-01T05:19:12.112Z

Updated: 2024-11-05T09:18:57.117Z

Reserved: 2024-02-19T14:20:24.201Z

Link: CVE-2024-26960

cve-icon Vulnrichment

Updated: 2024-08-02T00:21:06.048Z

cve-icon NVD

Status : Awaiting Analysis

Published: 2024-05-01T06:15:12.323

Modified: 2024-11-21T09:03:29.657

Link: CVE-2024-26960

cve-icon Redhat

Severity : Low

Publid Date: 2024-05-01T00:00:00Z

Links: CVE-2024-26960 - Bugzilla