Re: [tip: x86/urgent] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

From: Sasha Levin
Date: Sat Aug 24 2019 - 12:30:47 EST

On Fri, Aug 23, 2019 at 01:10:49AM -0000, tip-bot2 for Tom Lendacky wrote:
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: c49a0a80137c7ca7d6ced4c812c9e07a949f6f24
Author: Tom Lendacky <thomas.lendacky@xxxxxxx>
AuthorDate: Mon, 19 Aug 2019 15:52:35
Committer: Borislav Petkov <bp@xxxxxxx>
CommitterDate: Mon, 19 Aug 2019 19:42:52 +02:00

x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

There have been reports of RDRAND issues after resuming from suspend on
some AMD family 15h and family 16h systems. This issue stems from a BIOS
not performing the proper steps during resume to ensure RDRAND continues
to function properly.

RDRAND support is indicated by CPUID Fn00000001_ECX[30]. This bit can be
reset by clearing MSR C001_1004[62]. Any software that checks for RDRAND
support using CPUID, including the kernel, will believe that RDRAND is
not supported.

Update the CPU initialization to clear the RDRAND CPUID bit for any family
15h and 16h processor that supports RDRAND. If it is known that the family
15h or family 16h system does not have an RDRAND resume issue or that the
system will not be placed in suspend, the "rdrand=force" kernel parameter
can be used to stop the clearing of the RDRAND CPUID bit.

Additionally, update the suspend and resume path to save and restore the
MSR C001_1004 value to ensure that the RDRAND CPUID setting remains in
place after resuming from suspend.

Note, that clearing the RDRAND CPUID bit does not prevent a processor
that normally supports the RDRAND instruction from executing it. So any
code that determined the support based on family and model won't #UD.

Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Chen Yu <yu.c.chen@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: "linux-doc@xxxxxxxxxxxxxxx" <linux-doc@xxxxxxxxxxxxxxx>
Cc: "linux-pm@xxxxxxxxxxxxxxx" <linux-pm@xxxxxxxxxxxxxxx>
Cc: Nathan Chancellor <natechancellor@xxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Pavel Machek <pavel@xxxxxx>
Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "x86@xxxxxxxxxx" <x86@xxxxxxxxxx>
Documentation/admin-guide/kernel-parameters.txt | 7 +-
arch/x86/include/asm/msr-index.h | 1 +-
arch/x86/kernel/cpu/amd.c | 66 +------------
arch/x86/power/cpu.c | 86 ++--------------
4 files changed, 13 insertions(+), 147 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 4c19719..47d981a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4090,13 +4090,6 @@
Run specified binary instead of /init from the ramdisk,
used for early userspace startup. See initrd.

- rdrand= [X86]
- force - Override the decision by the kernel to hide the
- advertisement of RDRAND support (this affects
- certain AMD processors because of buggy BIOS
- support, specifically around the suspend/resume
- path).

Why is this being removed (along with supporting code)?