CVE-2020-12771

What it actually does

A deadlock occurs in btree_gc_coalesce when a coalescing operation fails, leaving the bcache subsystem unable to proceed and causing kernel thread hangs.

Why it matters to the business

Linux systems and storage appliances using bcache may experience prolonged unavailability or require reboots, disrupting workloads on servers and network-attached storage across many distributions and vendor products.

How an attack unfolds

A local attacker triggers failure paths during bcache garbage collection coalescing to induce the deadlock, resulting in denial of service without needing elevated privileges beyond local access.

How to fix it

Upgrade the Linux kernel to a version newer than 5.6.11 that contains the fix for the deadlock in drivers/md/bcache/btree.c. Apply the latest updates from distribution vendors and NetApp for all affected products.

How to confirm the fix worked

Confirm the running kernel version exceeds 5.6.11, review the btree.c source for corrected locking around coalescing, and exercise bcache workloads to verify absence of hangs.

Typical attack path

1. Obtain local shell access on an affected system
2. Perform or induce bcache operations that reach the coalescing code path
3. Force a failure condition in btree_gc_coalesce to trigger the deadlock

How to detect it

Inspect kernel logs and dmesg for messages about blocked tasks or bcache coalescing failures; correlate with system hangs on hosts running affected kernel versions.

• btree_gc_coalesce
• drivers/md/bcache/btree.c

What your security team should do

• Apply kernel updates immediately on any systems exposing bcache
• Audit bcache usage and disable where updates are delayed
• Monitor kernel logs for hung-task warnings related to bcache