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How the CVE Works:
A softlockup in the Linux kernel occurs when using shmem (shared memory) with mTHP (multi-size Transparent Huge Pages) swap. During a swapin operation, the kernel attempts to allocate a mTHP folio. If the readahead mechanism has already brought smaller order-0 folios into the swap cache for the same address range, the subsequent mTHP allocation fails with -EEXIST. The `shmem_swapin_folio` function enters an infinite retry loop, continuously attempting and failing to allocate the huge page. This loop consumes a CPU core completely, triggering the kernel’s softlockup detector and rendering the system unresponsive. The issue stems from improper handling of swap cache conflicts between different page sizes.
Platform: Linux Kernel
Version: Up to 6.15
Vulnerability: Softlockup
Severity: Critical
date: 07/09/2025
Prediction: Patch by Q4 2025
What Undercode Say:
echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled swapon /dev/zram0 echo 1G > /sys/fs/cgroup/test/memory.max cat /tmp/large_file > /dev/null
// Code path leading to softlockup shmem_swapin_folio+0x309/0xcf0 shmem_get_folio_gfp+0x2ed/0x5b0 shmem_file_read_iter+0x7f/0x2e0
How Exploit:
Trigger mTHP swapin
Cause swap cache conflict
Infinite retry loop
Protection from this CVE:
Update Kernel
Disable mTHP
Use standard pages
Impact:
System freeze
CPU exhaustion
Denial-of-service
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
Undercode
