{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2025-21693", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2024-12-29T08:45:45.742Z", "datePublished": "2025-02-10T15:58:49.056Z", "dateUpdated": "2026-08-05T11:53:39.821Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-08-05T11:53:39.821Z" }, "descriptions": [ { "lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: zswap: properly synchronize freeing resources during CPU hotunplug\n\nIn zswap_compress() and zswap_decompress(), the per-CPU acomp_ctx of the\ncurrent CPU at the beginning of the operation is retrieved and used\nthroughout. However, since neither preemption nor migration are disabled,\nit is possible that the operation continues on a different CPU.\n\nIf the original CPU is hotunplugged while the acomp_ctx is still in use,\nwe run into a UAF bug as some of the resources attached to the acomp_ctx\nare freed during hotunplug in zswap_cpu_comp_dead() (i.e. \nacomp_ctx.buffer, acomp_ctx.req, or acomp_ctx.acomp).\n\nThe problem was introduced in commit 1ec3b5fe6eec (\"mm/zswap: move to use\ncrypto_acomp API for hardware acceleration\") when the switch to the\ncrypto_acomp API was made. Prior to that, the per-CPU crypto_comp was\nretrieved using get_cpu_ptr() which disables preemption and makes sure the\nCPU cannot go away from under us. Preemption cannot be disabled with the\ncrypto_acomp API as a sleepable context is needed.\n\nUse the acomp_ctx.mutex to synchronize CPU hotplug callbacks allocating\nand freeing resources with compression/decompression paths. Make sure\nthat acomp_ctx.req is NULL when the resources are freed. In the\ncompression/decompression paths, check if acomp_ctx.req is NULL after\nacquiring the mutex (meaning the CPU was offlined) and retry on the new\nCPU.\n\nThe initialization of acomp_ctx.mutex is moved from the CPU hotplug\ncallback to the pool initialization where it belongs (where the mutex is\nallocated). In addition to adding clarity, this makes sure that CPU\nhotplug cannot reinitialize a mutex that is already locked by\ncompression/decompression.\n\nPreviously a fix was attempted by holding cpus_read_lock() [1]. This\nwould have caused a potential deadlock as it is possible for code already\nholding the lock to fall into reclaim and enter zswap (causing a\ndeadlock). A fix was also attempted using SRCU for synchronization, but\nJohannes pointed out that synchronize_srcu() cannot be used in CPU hotplug\nnotifiers [2].\n\nAlternative fixes that were considered/attempted and could have worked:\n- Refcounting the per-CPU acomp_ctx. This involves complexity in\n handling the race between the refcount dropping to zero in\n zswap_[de]compress() and the refcount being re-initialized when the\n CPU is onlined.\n- Disabling migration before getting the per-CPU acomp_ctx [3], but\n that's discouraged and is a much bigger hammer than needed, and could\n result in subtle performance issues.\n\n[1]https://lkml.kernel.org/20241219212437.2714151-1-yosryahmed@google.com/\n[2]https://lkml.kernel.org/20250107074724.1756696-2-yosryahmed@google.com/\n[3]https://lkml.kernel.org/20250107222236.2715883-2-yosryahmed@google.com/\n\n[yosryahmed@google.com: remove comment]" } ], "metrics": [ { "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH" }, "scenarios": [ { "lang": "en", "value": "AV:L - The UAF is reached through local memory-management activity — `madvise(MADV_PAGEOUT)`/reclaim driving `zswap_store()`→`zswap_compress()` and swap-in faults driving `zswap_load()`→`zswap_decompress()` — raced against a local CPU offline transition (sysfs `cpuN/online`, suspend/resume, or hypervisor vCPU hot-remove). No remote peer or network input reaches `mm/zswap.c`.\nAC:L - The attacker fully controls the zswap side (unbounded `MADV_PAGEOUT`/swap-in loops on threads pinned to each CPU via `sched_setaffinity()`), and the offline operation itself performs the migration that opens the window, since `CPUHP_MM_ZSWP_POOL_PREPARE` teardown runs only after the dying CPU's tasks have been pushed to other CPUs; the pre-fix per-CPU mutex additionally parks several attacker threads on the dying CPU's ctx so they all wake onto freed resources. Hotplug or suspend/resume cycles can be repeated indefinitely until the race lands, and zswap is a mainstream distro config (`CONFIG_ZSWAP`, `CONFIG_ZSWAP_DEFAULT_ON`).\nPR:L - The compress/decompress half needs no privilege at all — any user's own anonymous memory plus `madvise()` reaches it with no `capable()` check — and the offline half requires no elevated credentials in realistic deployments: the identical CPU-down sequence runs on every suspend/resume, which an unprivileged desktop/laptop/Android session can request via logind/polkit, and cloud guests (Xen vCPU availability, pseries DLPAR, s390, ACPI eject) and vendor Android hotplug governors offline CPUs autonomously while the attacker keeps zswap traffic in flight.\nUI:N - The attacker drives both the reclaim/swap-in traffic and the hotplug or suspend cycle itself; no victim action, mount, or file open is needed, and on hotplug-driven platforms the offline event occurs during normal operation.\nS:U - The freed `acomp_ctx` resources and the resulting corruption are entirely within the kernel's own memory and security authority, with no crossing of a VM, IOMMU, or sandbox boundary.\nC:H - `zswap_decompress()` memcpys into and then decompresses out of the freed 8 KB `acomp_ctx->buffer`, writing the result into the folio returned to userspace by the swap-in fault, so a groomed reallocation of that slab object leaks arbitrary kernel data into an attacker-readable page; the freed `acomp_req`/`crypto_acomp` are also read as live objects.\nI:H - `crypto_acomp_compress()` expands to `crypto_acomp_reqtfm(req)->compress(req)`, an indirect call through a function pointer loaded from the freed tfm, giving control-flow hijack after heap grooming; additionally the compressor writes up to `PAGE_SIZE * 2` of attacker-influenced output into the freed buffer and `acomp_request_set_params()` writes into the freed request.\nA:H - The use-after-free reliably oopses or panics the kernel, and `zswap_decompress()` carries two unconditional `BUG_ON()`s on the crypto result and `req->dlen` that fire immediately on garbage from freed structures — the bug was reported precisely as a crash by both Johannes Weiner and Sam Sun." } ] } ], "affected": [ { "product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "mm/zswap.c" ], "versions": [ { "version": "1ec3b5fe6eec782f4e5e0a80e4ce1909ffd5d161", "lessThan": "8d29ff5d50304daa41dc3cfdda4a9d1e46cf5be1", "status": "affected", "versionType": "git" }, { "version": "1ec3b5fe6eec782f4e5e0a80e4ce1909ffd5d161", "lessThan": "12dcb0ef540629a281533f9dedc1b6b8e14cfb65", "status": "affected", "versionType": "git" } ] }, { "product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "mm/zswap.c" ], "versions": [ { "version": "5.11", "status": "affected" }, { "version": "0", "lessThan": "5.11", "status": "unaffected", "versionType": "semver" }, { "version": "6.12.12", "lessThanOrEqual": "6.12.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.13", "lessThanOrEqual": "*", "status": "unaffected", "versionType": "original_commit_for_fix" } ] } ], "cpeApplicability": [ { "nodes": [ { "operator": "OR", "negate": false, "cpeMatch": [ { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.11", "versionEndExcluding": "6.12.12" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.11", "versionEndExcluding": "6.13" } ] } ] } ], "references": [ { "url": "https://git.kernel.org/stable/c/8d29ff5d50304daa41dc3cfdda4a9d1e46cf5be1" }, { "url": "https://git.kernel.org/stable/c/12dcb0ef540629a281533f9dedc1b6b8e14cfb65" } ], "title": "mm: zswap: properly synchronize freeing resources during CPU hotunplug", "x_generator": { "engine": "bippy-1.2.0" } }, "adp": [ { "metrics": [ { "cvssV3_1": { "scope": "UNCHANGED", "version": "3.1", "baseScore": 7.8, "attackVector": "LOCAL", "baseSeverity": "HIGH", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "integrityImpact": "HIGH", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "HIGH", "privilegesRequired": "LOW", "confidentialityImpact": "HIGH" } }, { "other": { "type": "ssvc", "content": { "id": "CVE-2025-21693", "role": "CISA Coordinator", "options": [ { "Exploitation": "none" }, { "Automatable": "no" }, { "Technical Impact": "total" } ], "version": "2.0.3", "timestamp": "2025-02-10T17:10:07.856772Z" } } } ], "problemTypes": [ { "descriptions": [ { "lang": "en", "type": "CWE", "cweId": "CWE-416", "description": "CWE-416 Use After Free" } ] } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2025-02-10T17:11:03.608Z" } } ] } }