IM
IronMonkey Threat Research

CVE-2024-26960 HIGH

Published: 2024-05-01 | Last Modified: 2026-08-04 | Status: Modified

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<-----

Additional Descriptions (1)

En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: mm: swap: corrige la ejecución entre free_swap_and_cache() y swapoff() Anteriormente existía una ventana teórica donde swapoff() podía ejecutar y desmantelar un swap_info_struct mientras se realizaba una llamada a free_swap_and_cache(). corriendo en otro hilo. Esto podría causar, entre otras malas posibilidades, que swap_page_trans_huge_swapped() (llamado por free_swap_and_cache()) acceda a la memoria liberada para swap_map. Este es un problema teórico y no he podido provocarlo a partir de un caso de prueba. Pero ha habido un acuerdo basado en la revisión del código de que esto es posible (ver enlace a continuación). Solucionarlo usando get_swap_device()/put_swap_device(), lo que detendrá swapoff(). Hubo una verificación adicional en _swap_info_get() para confirmar que la entrada de intercambio no era gratuita. Esto no está presente en get_swap_device() porque en general no tiene sentido debido a la ejecución entre obtener la referencia y el intercambio. Así que agregué una verificación equivalente directamente en free_swap_and_cache(). Detalles de cómo provocar un posible problema (gracias a David Hildenbrand por derivar esto): --8&lt;----- __swap_entry_free() podría ser el último usuario y dar como resultado "count == SWAP_HAS_CACHE". swapoff-&gt;try_to_unuse() se detendrá tan pronto como si-&gt;inuse_pages==0. Entonces la pregunta es: ¿alguien podría reclamar la publicación y activar si-&gt;inuse_pages==0, antes de que completemos swap_page_trans_huge_swapped()? Imagine lo siguiente: folio de 2 MiB en el swapcache. Sólo 2 subpáginas siguen siendo referencias mediante entradas de intercambio. El proceso 1 todavía hace referencia a la subpágina 0 mediante la entrada de intercambio. El proceso 2 todavía hace referencia a la subpágina 1 mediante la entrada de intercambio. El proceso 1 se cierra. Llama a free_swap_and_cache(). -&gt; count == SWAP_HAS_CACHE [luego, adelantado en el hipervisor, etc.] El proceso 2 se cierra. Llama a free_swap_and_cache(). -&gt; count == SWAP_HAS_CACHE El proceso 2 continúa, pasa swap_page_trans_huge_swapped() y llama a __try_to_reclaim_swap(). __try_to_reclaim_swap()-&gt;folio_free_swap()-&gt;delete_from_swap_cache()-&gt; put_swap_folio()-&gt;free_swap_slot()-&gt;swapcache_free_entries()-&gt; swap_entry_free()-&gt;swap_range_free()-&gt; ... WRITE_ONCE(si-&gt;inuse_pages, si-&gt;inuse_pages - nr_entries); ¿Qué impide que el intercambio tenga éxito después de que el proceso 2 recuperó el caché de intercambio pero antes de que el proceso 1 terminara su llamada a swap_page_trans_huge_swapped()? --8&lt;-----

CVSS Metrics

Base Score: 7.0 (HIGH)

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H

Attack VectorLOCAL
Attack ComplexityHIGH
Privileges RequiredLOW
User InteractionNONE
ScopeUNCHANGED
Confidentiality ImpactHIGH
Integrity ImpactHIGH
Availability ImpactHIGH

Source: 416baaa9-dc9f-4396-8d5f-8c081fb06d67

Type: Secondary

Exploitability Score: 1.0

Impact Score: 5.9

Weaknesses

Source Type Description
134c704f-9b21-4f2e-91b3-4a467353bcc0 Secondary
en CWE-362

Affected Products

Vendor Product Version Update Type
linux linux_kernel * <built-in method update of dict object at 0x7fd39c47ae40> Operating System
linux linux_kernel * <built-in method update of dict object at 0x7fd340a8d080> Operating System
linux linux_kernel * <built-in method update of dict object at 0x7fd39c2a7b80> Operating System
linux linux_kernel * <built-in method update of dict object at 0x7fd3688e6940> Operating System
linux linux_kernel * <built-in method update of dict object at 0x7fd39c47a040> Operating System
linux linux_kernel * <built-in method update of dict object at 0x7fd39c479180> Operating System
debian debian_linux 10.0 <built-in method update of dict object at 0x7fd36a653cc0> Operating System

Affected Configurations

Operator: OR

Vulnerable CPE
Yes cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Yes cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Yes cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Yes cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Yes cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Yes cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*

Operator: OR

Vulnerable CPE
Yes cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*

References