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.
Metrics
Affected Vendors & Products
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| Linux |
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References
History
Mon, 20 Jul 2026 14:45:00 +0000
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|---|---|---|
| Metrics |
cvssV3_1
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Sun, 19 Jul 2026 15:30:00 +0000
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| 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. | |
| Title | zram: fix use-after-free in zram_writeback_endio | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-07-20T13:41:50.089Z
Reserved: 2026-07-19T07:54:57.023Z
Link: CVE-2026-63951
Vulnrichment
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NVD
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Redhat
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OpenCVE Enrichment
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