{ "schema_version": "1.6.1", "id": "RPI-CVE-2024-26960", "modified": "2026-10-09T11:39:44Z", "upstream": [ "CVE-2024-26960" ], "published": "2024-05-01T05:22:01Z", "summary": "mm: swap: fix race between free_swap_and_cache() and swapoff()", "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: swap: fix race between free_swap_and_cache() and swapoff()\n\nThere was previously a theoretical window where swapoff() could run and\nteardown a swap_info_struct while a call to free_swap_and_cache() was\nrunning in another thread. This could cause, amongst other bad\npossibilities, swap_page_trans_huge_swapped() (called by\nfree_swap_and_cache()) to access the freed memory for swap_map.\n\nThis is a theoretical problem and I haven't been able to provoke it from a\ntest case. But there has been agreement based on code review that this is\npossible (see link below).\n\nFix it by using get_swap_device()/put_swap_device(), which will stall\nswapoff(). There was an extra check in _swap_info_get() to confirm that\nthe swap entry was not free. This isn't present in get_swap_device()\nbecause it doesn't make sense in general due to the race between getting\nthe reference and swapoff. So I've added an equivalent check directly in\nfree_swap_and_cache().\n\nDetails of how to provoke one possible issue (thanks to David Hildenbrand\nfor deriving this):\n\n--8<-----\n\n__swap_entry_free() might be the last user and result in\n\"count == SWAP_HAS_CACHE\".\n\nswapoff->try_to_unuse() will stop as soon as soon as si->inuse_pages==0.\n\nSo the question is: could someone reclaim the folio and turn\nsi->inuse_pages==0, before we completed swap_page_trans_huge_swapped().\n\nImagine the following: 2 MiB folio in the swapcache. Only 2 subpages are\nstill references by swap entries.\n\nProcess 1 still references subpage 0 via swap entry.\nProcess 2 still references subpage 1 via swap entry.\n\nProcess 1 quits. Calls free_swap_and_cache().\n-> count == SWAP_HAS_CACHE\n[then, preempted in the hypervisor etc.]\n\nProcess 2 quits. Calls free_swap_and_cache().\n-> count == SWAP_HAS_CACHE\n\nProcess 2 goes ahead, passes swap_page_trans_huge_swapped(), and calls\n__try_to_reclaim_swap().\n\n__try_to_reclaim_swap()->folio_free_swap()->delete_from_swap_cache()->\nput_swap_folio()->free_swap_slot()->swapcache_free_entries()->\nswap_entry_free()->swap_range_free()->\n...\nWRITE_ONCE(si->inuse_pages, si->inuse_pages - nr_entries);\n\nWhat stops swapoff to succeed after process 2 reclaimed the swap cache\nbut before process1 finished its call to swap_page_trans_huge_swapped()?\n\n--8<-----", "severity": [ { "type": "CVSS_V3", "score": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H" } ], "affected": [ { "package": { "ecosystem": "Raspberry Pi OS:12", "name": "linux" }, "versions": [ "1:6.1.21-1+rpt1", "1:6.1.47-1+rpt2", "1:6.1.47-1+rpt4", "1:6.1.54-1+rpt2", "1:6.1.58-1+rpt2", "1:6.1.63-1+rpt1", "1:6.1.73-1+rpt1", "1:6.6.20-1+rpt1" ], "ecosystem_specific": { "binaries": [ { "binary_name": "linux-image-6.1.0-rpi1-rpi-v6", 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"https://www.cve.org/CVERecord?id=CVE-2024-26960" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/d85c11c97ecf92d47a4b29e3faca714dc1f18d0d" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/2da5568ee222ce0541bfe446a07998f92ed1643e" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/1ede7f1d7eed1738d1b9333fd1e152ccb450b86a" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/0f98f6d2fb5fad00f8299b84b85b6bc1b6d7d19a" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/3ce4c4c653e4e478ecb15d3c88e690f12cbf6b39" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/363d17e7f7907c8e27a9e86968af0eaa2301787b" }, { "type": "FIX", "url": "https://git.kernel.org/stable/c/82b1c07a0af603e3c47b906c8e991dc96f01688e" } ], "database_specific": { "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git", "upstream_modified": "2026-08-05T11:52:36Z", "severity": { "source": "kernel CNA", "cvss_v3_score": 7.0, "cvss_v3_severity": "High", "scenario": "AV:L - The vulnerable path is reached only through local memory-management syscalls \u2014 `munmap`/`exit` (`zap_pte_range` \u2192 mm/memory.c:1636), `madvise(MADV_DONTNEED/MADV_FREE)` (mm/madvise.c:659), and tmpfs truncate/punch-hole (mm/shmem.c:842). No remote or adjacent-network consumer reaches `free_swap_and_cache()`.\nAC:H - The attacker controls only their half of the race; the other half is `swapoff()`, gated by `capable(CAP_SYS_ADMIN)` in the *initial* user namespace (mm/swapfile.c:2411, `capable()` not `ns_capable()`), so it cannot be induced from a user namespace and must be performed by an administrator or system script. Landing the hit additionally requires being preempted inside a sub-microsecond window between `__swap_entry_free()` and `swap_page_trans_huge_swapped()` while a second process reclaims the last swapcache reference \u2014 the author explicitly could not provoke it from a test case.\nPR:L - The attacker-controlled side needs nothing beyond an ordinary unprivileged local account: any process can spin `munmap`/`madvise`/tmpfs-punch loops over swapped-out anonymous pages to drive `free_swap_and_cache()` at high rate. No capability, namespace, or filesystem privilege is required for that half.\nUI:N - No victim is tricked into any action; the required `swapoff()` is routine system activity (systemd swap-unit deactivation, hibernate scripts, Android zram reconfiguration) that occurs without human participation, and that environmental dependency is already accounted for in AC:H rather than counted twice as UI:R.\nS:U - The corruption is confined to kernel data structures within the kernel's own security authority \u2014 standard local kernel memory corruption / privilege escalation. No VM, hypervisor, or IOMMU boundary is crossed.\nC:H - Use-after-free reads of the vfree'd `swap_map`, the kvfree'd `cluster_info`, and a freed `struct address_space` (XArray walk in `filemap_get_folio()`) can disclose reallocated kernel memory. Additionally, `swapoff`+`swapon` reuses the `swap_info_struct` slot, so the stale `free_swap_and_cache()` corrupts the *new* device's counts and prematurely frees slots still referenced by other processes, handing a victim's swapped-out anonymous data to the attacker on swap-in.\nI:H - `lock_cluster()` performs `spin_lock(&ci->lock)` on the kvfree'd `cluster_info` \u2014 for typical swap sizes a kmalloc-slab object \u2014 giving a sprayable use-after-free *write* primitive, and `__try_to_reclaim_swap()` manipulates folio locks/refcounts derived from a freed `address_space`. Combined with silent swap-map count corruption after `swapon` reuse, this is exploitable for control-flow hijack and cross-process data modification.\nA:H - The most likely immediate outcome is a kernel oops \u2014 dereferencing a NULLed `si->swap_map`, faulting in the unmapped vfree'd vmalloc range, a NULL `swapper_spaces[type]`, or tripping `VM_BUG_ON(count != SWAP_HAS_CACHE)` in `__swap_entry_free_locked()`. Any of these panics the kernel." } } }