{
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  "id": "RPI-CVE-2025-21693",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2025-21693"
  ],
  "published": "2025-02-10T16:00:58Z",
  "summary": "mm: zswap: properly synchronize freeing resources during CPU hotunplug",
  "details": "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]",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21693"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/8d29ff5d50304daa41dc3cfdda4a9d1e46cf5be1"
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      "source": "kernel CNA",
      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The UAF is reached through local memory-management activity \u2014 `madvise(MADV_PAGEOUT)`/reclaim driving `zswap_store()`\u2192`zswap_compress()` and swap-in faults driving `zswap_load()`\u2192`zswap_decompress()` \u2014 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 \u2014 any user's own anonymous memory plus `madvise()` reaches it with no `capable()` check \u2014 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 \u2014 the bug was reported precisely as a crash by both Johannes Weiner and Sam Sun."
    }
  }
}