# CVE-2026-63951: zram: fix use-after-free in zram\_writeback\_endio

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Published: 2026-07-19 · Updated: 2026-08-05

## Affected

- Linux: version range not reported
- Linux: 6.19

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
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  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-63951

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/ebe2cbefc86291fa7f386447a81995640df4e2fd
- https://git.kernel.org/stable/c/bf62f69574b19720ae5fbbbcdf24a0c4e3e05e43

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