FKIE_CVE-2026-63951

Vulnerability from fkie_nvd - Published: 2026-07-19 16:17 - Updated: 2026-07-20 15:16
Severity
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: zram: fix use-after-free in zram_writeback_endio A crash was observed in zram_writeback_endio due to a NULL pointer dereference in wake_up. The root cause is a race condition between the bio completion handler (zram_writeback_endio) and the writeback task. In zram_writeback_endio, wake_up() is called on &wb_ctl->done_wait after releasing wb_ctl->done_lock. This creates a race window where the writeback task can see num_inflight become 0, return, and free wb_ctl before zram_writeback_endio calls wake_up(). CPU 0 (zram_writeback_endio) CPU 1 (writeback_store) ============================ ============================ zram_writeback_slots zram_submit_wb_request zram_submit_wb_request wait_event(wb_ctl->done_wait) spin_lock(&wb_ctl->done_lock); list_add(&req->entry, &wb_ctl->done_reqs); spin_unlock(&wb_ctl->done_lock); wake_up(&wb_ctl->done_wait); zram_complete_done_reqs spin_lock(&wb_ctl->done_lock); list_add(&req->entry, &wb_ctl->done_reqs); spin_unlock(&wb_ctl->done_lock); while (num_inflight) > 0) spin_lock(&wb_ctl->done_lock); list_del(&req->entry); spin_unlock(&wb_ctl->done_lock); // num_inflight becomes 0 atomic_dec(num_inflight); // Leave zram_writeback_slots // Free wb_ctl release_wb_ctl(wb_ctl); // UAF crash! wake_up(&wb_ctl->done_wait); This patch fixes this race by using RCU. By protecting wb_ctl with rcu_read_lock() in zram_writeback_endio and using kfree_rcu() to free it, we ensure that wb_ctl remains valid during the execution of zram_writeback_endio.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "drivers/block/zram/zram_drv.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "ebe2cbefc86291fa7f386447a81995640df4e2fd",
              "status": "affected",
              "version": "f405066a1f0db818270f49a5e96be329bcabde1e",
              "versionType": "git"
            },
            {
              "lessThan": "bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43",
              "status": "affected",
              "version": "f405066a1f0db818270f49a5e96be329bcabde1e",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "drivers/block/zram/zram_drv.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.19"
            },
            {
              "lessThan": "6.19",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.0.*",
              "status": "unaffected",
              "version": "7.0.12",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nzram: fix use-after-free in zram_writeback_endio\n\nA crash was observed in zram_writeback_endio due to a NULL pointer\ndereference in wake_up.  The root cause is a race condition between the\nbio completion handler (zram_writeback_endio) and the writeback task.\n\nIn zram_writeback_endio, wake_up() is called on \u0026wb_ctl-\u003edone_wait after\nreleasing wb_ctl-\u003edone_lock.  This creates a race window where the\nwriteback task can see num_inflight become 0, return, and free wb_ctl\nbefore zram_writeback_endio calls wake_up().\n\nCPU 0 (zram_writeback_endio)     CPU 1 (writeback_store)\n============================     ============================\n                                 zram_writeback_slots\n                                   zram_submit_wb_request\n                                   zram_submit_wb_request\n                                   wait_event(wb_ctl-\u003edone_wait)\nspin_lock(\u0026wb_ctl-\u003edone_lock);\nlist_add(\u0026req-\u003eentry, \u0026wb_ctl-\u003edone_reqs);\nspin_unlock(\u0026wb_ctl-\u003edone_lock);\nwake_up(\u0026wb_ctl-\u003edone_wait);\n                                   zram_complete_done_reqs\nspin_lock(\u0026wb_ctl-\u003edone_lock);\nlist_add(\u0026req-\u003eentry, \u0026wb_ctl-\u003edone_reqs);\nspin_unlock(\u0026wb_ctl-\u003edone_lock);\n                                   while (num_inflight) \u003e 0)\n                                     spin_lock(\u0026wb_ctl-\u003edone_lock);\n                                     list_del(\u0026req-\u003eentry);\n                                     spin_unlock(\u0026wb_ctl-\u003edone_lock);\n                                     // num_inflight becomes 0\n                                     atomic_dec(num_inflight);\n\n                                 // Leave zram_writeback_slots\n                                 // Free wb_ctl\n                                 release_wb_ctl(wb_ctl);\n// UAF crash!\nwake_up(\u0026wb_ctl-\u003edone_wait);\n\nThis patch fixes this race by using RCU.  By protecting wb_ctl with\nrcu_read_lock() in zram_writeback_endio and using kfree_rcu() to free it,\nwe ensure that wb_ctl remains valid during the execution of\nzram_writeback_endio."
    }
  ],
  "id": "CVE-2026-63951",
  "lastModified": "2026-07-20T15:16:58.717",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "LOCAL",
          "availabilityImpact": "HIGH",
          "baseScore": 7.8,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 1.8,
        "impactScore": 5.9,
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-07-19T16:17:13.837",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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