CVE-2024-35875 x86/coco: Require seeding RNG with RDRAND on CoCo systems
Linux kernel CoCo systems may boot with a weak RNG if RDRAND fails, compromising cryptographic security against malicious hosts.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2024-35875?
CVE-2024-35875 is a high-severity vulnerability in Linux kernel, published on 19 May 2024. 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/coco: Require seeding RNG with RDRAND on CoCo systems
There are few uses of CoCo that don't rely on working cryptography and hence a working RNG. Unfortunately, the CoCo threat model means that the VM host cannot be trusted and may actively work against guests to extract secrets or manipulate computation. Since a malicious host can modify or observe nearly all inputs to guests, the only remaining source of entropy for CoCo guests is RDRAND.
If RDRAND is broken -- due to CPU hardware fault -- the RNG as a whole is meant to gracefully continue on gathering entropy from other sources, but since there aren't other sources on CoCo, this is catastrophic. This is mostly a concern at boot time when initially seeding the RNG, as after that the consequences of a broken RDRAND are much more theoretical.
So, try at boot to seed the RNG using 256 bits of RDRAND output. If this fails, panic(). This will also trigger if the system is booted without RDRAND, as RDRAND is essential for a safe CoCo boot.
Add this deliberately to be "just a CoCo x86 driver feature" and not part of the RNG itself. Many device drivers and platforms have some desire to contribute something to the RNG, and add_device_randomness() is specifically meant for this purpose.
Any driver can call it with seed data of any quality, or even garbage quality, and it can only possibly make the quality of the RNG better or have no effect, but can never make it worse.
Rather than trying to build something into the core of the RNG, consider the particular CoCo issue just a CoCo issue, and therefore separate it all out into driver (well, arch/platform) code.
[ bp: Massage commit message. ]
Which versions are affected by CVE-2024-35875?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-35875?
Apply the vendor's patch.
Is CVE-2024-35875 being exploited?
There is no known exploitation of CVE-2024-35875 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2024-35875, DEBIAN-CVE-2024-35875, GHSA-43j5-9vgx-7g7j, ROOT-OS-DEBIAN-11-CVE-2024-35875, ROOT-OS-UBUNTU-2204-CVE-2024-35875, UBUNTU-CVE-2024-35875.