GHSA-JMWM-428X-W863
Vulnerability from github – Published: 2026-07-19 18:31 – Updated: 2026-07-20 15:31In 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.
{
"affected": [],
"aliases": [
"CVE-2026-63951"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-07-19T16:17:13Z",
"severity": "HIGH"
},
"details": "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": "GHSA-jmwm-428x-w863",
"modified": "2026-07-20T15:31:53Z",
"published": "2026-07-19T18:31:47Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63951"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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