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When a process uses vfork() or the CLONE_VM flag, the kernel’s Memory Management Context ID (MMCID) logic must properly fix up the newly created task’s CID. The vulnerability arises when the number of tasks associated to a process is smaller than the number of MMCID users. The broken logic walks the task list to find the vfork()ed task but incorrectly accounts already processed tasks as well. If this double processing reduces the count of “to be handled” tasks to zero, the walk stops prematurely. Consequently, the vfork()ed task’s CID is not fixed up. Later, when the scheduler tries to schedule in that task, it calls mm_get_cid() to acquire a transitional CID. Because the CID was never fixed, the function enters an endless loop, causing the machine to stall completely. The fix removes the faulty accounting condition and ensures the fixup always walks the full task list when the exact number of users cannot be found.
dailycve form:
Platform: Linux kernel
Version: 6.19.x
Vulnerability : MMCID fixup flaw
Severity: Medium
date: 2026-05-08
Prediction: Patch available 2026-05-08
What Undercode Say:
Check current kernel version
uname -r
Test if running a vulnerable 6.19.x kernel
if [[ $(uname -r) =~ ^6.19.[0-8]$ ]]; then echo “VULNERABLE to CVE-2026-43417”; fi
Apply the fix by upgrading to 6.19.9 or 7.0+
sudo apt update && sudo apt upgrade linux-image-$(uname -r) Debian/Ubuntu
sudo dnf update kernel Fedora/RHEL
Alternatively, manually backport the patch commits:
git.kernel.org/stable/c/e6761cdce78a8919a537989afb6aaf6881469f83
git.kernel.org/stable/c/28b5a1395036d6c7a6c8034d85ad3d7d365f192c
Exploit:
An unprivileged user can repeatedly fork processes using vfork() or CLONE_VM flags, exhausting the MMCID fixup path. By carefully timing the creation of tasks such that the flawed counter reaches zero, the fixup for the vfork()ed task is skipped. This triggers an infinite loop in mm_get_cid() during the next schedule-in, effectively freezing the CPU core. A local attacker can cause a persistent denial-of-service by locking up kernel scheduling on the affected system.
Protection from this CVE
Upgrade to Linux kernel version 6.19.9 or 7.0 or any later release where the patch is applied. If an immediate upgrade is not possible, backport the two specific commits referenced in the official announcement. No known workarounds exist; the flaw is in core scheduling logic and requires a kernel update to resolve.
Impact
A local unprivileged attacker can cause a complete system stall (kernel hang) by exploiting the broken vfork/CLONE_VM CID fixup. The attack leads to denial of service, as the affected CPU core stops responding. Since the vulnerability resides in core scheduler code, it may also be triggered inadvertently by legitimate applications, leading to unexpected system unavailability.
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Sources:
Reported By: nvd.nist.gov
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