{
    "dataType": "CVE_RECORD",
    "dataVersion": "5.2",
    "cveMetadata": {
        "cveId": "CVE-2026-63951",
        "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "state": "PUBLISHED",
        "assignerShortName": "Linux",
        "dateReserved": "2026-07-19T07:54:57.023Z",
        "datePublished": "2026-07-19T14:55:44.105Z",
        "dateUpdated": "2026-08-05T12:37:36.330Z"
    },
    "containers": {
        "cna": {
            "providerMetadata": {
                "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
                "shortName": "Linux",
                "dateUpdated": "2026-08-05T12:37:36.330Z"
            },
            "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 &wb_ctl->done_wait after\nreleasing wb_ctl->done_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->done_wait)\nspin_lock(&wb_ctl->done_lock);\nlist_add(&req->entry, &wb_ctl->done_reqs);\nspin_unlock(&wb_ctl->done_lock);\nwake_up(&wb_ctl->done_wait);\n                                   zram_complete_done_reqs\nspin_lock(&wb_ctl->done_lock);\nlist_add(&req->entry, &wb_ctl->done_reqs);\nspin_unlock(&wb_ctl->done_lock);\n                                   while (num_inflight) > 0)\n                                     spin_lock(&wb_ctl->done_lock);\n                                     list_del(&req->entry);\n                                     spin_unlock(&wb_ctl->done_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(&wb_ctl->done_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": {
                        "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 vulnerable code runs only in the zram writeback path (`writeback_store` → `zram_writeback_slots` → `zram_submit_wb_request` → `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—the highest-impact deployment—an 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'd 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."
                        }
                    ]
                }
            ],
            "affected": [
                {
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/block/zram/zram_drv.c"
                    ],
                    "versions": [
                        {
                            "version": "f405066a1f0db818270f49a5e96be329bcabde1e",
                            "lessThan": "ebe2cbefc86291fa7f386447a81995640df4e2fd",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "f405066a1f0db818270f49a5e96be329bcabde1e",
                            "lessThan": "bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                },
                {
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/block/zram/zram_drv.c"
                    ],
                    "versions": [
                        {
                            "version": "6.19",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "6.19",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "7.0.12",
                            "lessThanOrEqual": "7.0.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "7.1",
                            "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": "6.19",
                                    "versionEndExcluding": "7.0.12"
                                },
                                {
                                    "vulnerable": true,
                                    "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                                    "versionStartIncluding": "6.19",
                                    "versionEndExcluding": "7.1"
                                }
                            ]
                        }
                    ]
                }
            ],
            "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"
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    }
}