# CVE-2026-63951: linux: In the Linux kernel, the following vulnerability has been resolved:  zram: fix use-after-free in zram_writeback_endio...

CVE-2026-63951 is fixed in linux on 1 platform by DEBIAN-CVE-2026-63951 (version 7.0.12-1), released July 20, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-63951
- Priority: High
- CVSS: 7.8 (high)
- EPSS: 0.0018
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 19, 2026

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 14 | 7.0.12-1 | [DEBIAN-CVE-2026-63951](https://patchblox.com/security/data/fix/linux/debian-cve-2026-63951.md) | Jul 20, 2026 |

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Data as of 2026-11-11T01:43:54Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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