CVE-2024-56761 x86/fred: Clear WFE in missing-ENDBRANCH #CPs
A flaw in the Linux kernel's FRED implementation causes a CPU dead loop during specific control-flow protection faults, leading to system denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-56761?
CVE-2024-56761 is a medium-severity denial of service vulnerability in Linux kernel, published on 6 Jan 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
x86/fred: Clear WFE in missing-ENDBRANCH #CPs
An indirect branch instruction sets the CPU indirect branch tracker (IBT) into WAIT_FOR_ENDBRANCH (WFE) state and WFE stays asserted across the instruction boundary. When the decoder finds an inappropriate instruction while WFE is set ENDBR, the CPU raises a #CP fault.
For the "kernel IBT no ENDBR" selftest where #CPs are deliberately triggered, the WFE state of the interrupted context needs to be cleared to let execution continue. Otherwise when the CPU resumes from the instruction that just caused the previous #CP, another missing-ENDBRANCH #CP is raised and the CPU enters a dead loop.
This is not a problem with IDT because it doesn't preserve WFE and IRET doesn't set WFE. But FRED provides space on the entry stack (in an expanded CS area) to save and restore the WFE state, thus the WFE state is no longer clobbered, so software must clear it.
Clear WFE to avoid dead looping in ibt_clear_fred_wfe() and the !ibt_fatal code path when execution is allowed to continue.
Clobbering WFE in any other circumstance is a security-relevant bug.
[ dhansen: changelog rewording ]
Which versions are affected by CVE-2024-56761?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-56761?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2024-56761 being exploited?
There is no known exploitation of CVE-2024-56761 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-55029, BELL-CVE-2024-56761, DEBIAN-CVE-2024-56761, GHSA-q9hp-vwf8-w58f, UBUNTU-CVE-2024-56761.