Vulnerability CVE-2024-26960
Published: 2024-05-01

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<-----

 References:
https://git.kernel.org/stable/c/d85c11c97ecf92d47a4b29e3faca714dc1f18d0d
https://git.kernel.org/stable/c/2da5568ee222ce0541bfe446a07998f92ed1643e
https://git.kernel.org/stable/c/1ede7f1d7eed1738d1b9333fd1e152ccb450b86a
https://git.kernel.org/stable/c/0f98f6d2fb5fad00f8299b84b85b6bc1b6d7d19a
https://git.kernel.org/stable/c/3ce4c4c653e4e478ecb15d3c88e690f12cbf6b39
https://git.kernel.org/stable/c/363d17e7f7907c8e27a9e86968af0eaa2301787b
https://git.kernel.org/stable/c/82b1c07a0af603e3c47b906c8e991dc96f01688e
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