CVE-2024-35875

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. ]