CVE-2024-26960

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<-----