GCVE Workshop - 22 September 2026 (14:00-18:00), Luxembourg Before The Vulnopticon Conference - Registration

CVE-2026-63951 (GCVE-0-2026-63951)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-08-05 12:37
VLAI
Title
zram: fix use-after-free in zram_writeback_endio
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.
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: f405066a1f0db818270f49a5e96be329bcabde1e , < ebe2cbefc86291fa7f386447a81995640df4e2fd (git)
Affected: f405066a1f0db818270f49a5e96be329bcabde1e , < bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43 (git)
guessed Create a notification for this product.
Linux Linux Affected: 6.19
Unaffected: 0 , < 6.19 (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
To clipboard 

{
  "containers": {
    "cna": {
      "affected": [
        {
          "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"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.0.12",
                  "versionStartIncluding": "6.19",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.1",
                  "versionStartIncluding": "6.19",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "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."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The vulnerable code runs only in the zram writeback path (`writeback_store` \u2192 `zram_writeback_slots` \u2192 `zram_submit_wb_request` \u2192 `zram_writeback_endio`), reachable via local sysfs writes or system daemons that invoke the same sysfs interface; there is no network, Bluetooth, or physical-device entry point.\nAC:L - This is a use-after-free race between batched bio completion handlers and writeback teardown; with default `writeback_batch_size` of 32 the attacker can drive concurrent inflight bios and repeatedly trigger writeback (directly as root or indirectly on Android via memory-pressure-driven `mmd` maintenance), controlling both sides of the race.\nPR:L - On Android phones\u2014the highest-impact deployment\u2014an unprivileged app can induce memory pressure that causes the memory management daemon to invoke `/sys/block/zram0/writeback` without attacker root; alternatively a local admin can write the sysfs attribute directly, but no capability beyond a basic local user is required in the primary mobile scenario.\nUI:N - Exploitation requires no victim interaction beyond normal system operation; automated writeback on memory pressure or attacker-initiated sysfs writeback proceeds without the user mounting filesystems, opening files, or taking deliberate action.\nS:U - Successful exploitation corrupts kernel heap memory and can lead to privilege escalation or kernel crash, but does not cross a VM, IOMMU, or sandbox security boundary beyond standard kernel compromise.\nC:H - The bug is a slab use-after-free on `struct zram_wb_ctl`; `wake_up()` operates on a freed `wait_queue_head_t`, enabling arbitrary kernel memory reads through heap reuse and controlled corruption of wait-queue state.\nI:H - Use-after-free on a kmalloc\u0027d control structure allows heap spraying and corruption of `wait_queue_head_t` and adjacent slab objects, providing a plausible path to arbitrary kernel writes and code execution.\nA:H - A production crash was already observed as a NULL pointer dereference in `wake_up()` during the race, and the underlying use-after-free can reliably cause kernel oops/panic even when not fully weaponized for code execution."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:37:36.330Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd"
        },
        {
          "url": "https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43"
        }
      ],
      "title": "zram: fix use-after-free in zram_writeback_endio",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63951",
    "datePublished": "2026-07-19T14:55:44.105Z",
    "dateReserved": "2026-07-19T07:54:57.023Z",
    "dateUpdated": "2026-08-05T12:37:36.330Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-63951",
      "date": "2026-09-04",
      "epss": "0.00165",
      "percentile": "0.05979"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-63951\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-07-19T16:17:13.837\",\"lastModified\":\"2026-07-27T17:44:23.777\",\"vulnStatus\":\"Awaiting Analysis\",\"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.\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"drivers/block/zram/zram_drv.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"f405066a1f0db818270f49a5e96be329bcabde1e\",\"lessThan\":\"ebe2cbefc86291fa7f386447a81995640df4e2fd\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"f405066a1f0db818270f49a5e96be329bcabde1e\",\"lessThan\":\"bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43\",\"versionType\":\"git\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"drivers/block/zram/zram_drv.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"6.19\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"6.19\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.0.12\",\"lessThanOrEqual\":\"7.0.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.1\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"type\":\"Secondary\",\"cvssData\":{\"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\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"LOW\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"HIGH\",\"integrityImpact\":\"HIGH\",\"availabilityImpact\":\"HIGH\"},\"exploitabilityScore\":1.8,\"impactScore\":5.9}]},\"references\":[{\"url\":\"https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"},{\"url\":\"https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\"}]}}",
    "redhat_vex": {
      "aggregate_severity": "None",
      "current_release_date": "2026-07-21T22:26:38+00:00",
      "cve": "CVE-2026-63951",
      "id": "CVE-2026-63951",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_not_affected": "274",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: zram: fix use-after-free in zram_writeback_endio",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63951.json",
      "version": "3"
    }
  }
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.