Subject: CVE-2024-35875: x86/coco: Require seeding RNG with RDRAND on CoCo systems

From: Greg Kroah-Hartman <[email-masked]>

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. ]

The Linux kernel CVE team has assigned CVE-2024-35875 to this issue.

Affected and fixed versions
===========================

	Fixed in 6.1.85 with commit 22943e4fe4b3
	Fixed in 6.6.26 with commit 453b5f2dec27
	Fixed in 6.8.5 with commit 08044b08b375
	Fixed in 6.9 with commit 99485c4c026f

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35875
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/coco/core.c
	arch/x86/include/asm/coco.h
	arch/x86/kernel/setup.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/22943e4fe4b3a2dcbadc3d38d5bf840bbdbfe374
	https://git.kernel.org/stable/c/453b5f2dec276c1bb4ea078bf8c0da57ee4627e5
	https://git.kernel.org/stable/c/08044b08b37528b82f70a87576c692b4e4b7716e
	https://git.kernel.org/stable/c/99485c4c026f024e7cb82da84c7951dbe3deb584