{"cve":"CVE-2026-63951","nvd":{"published":"2026-07-19T16:17:13.837","modified":"2026-10-30T14:32:36.153","status":"Analyzed","description":"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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","score":7.8},"cwes":["CWE-416"],"references":["https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43","https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd"],"products":["linux:linux_kernel"],"source":"nvd","synced_at":"2026-10-30T14:49:48.263840089+00:00"},"epss":{"cve":"CVE-2026-63951","score":0.00177,"percentile":0.07255,"date":"2026-10-30"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}