x86/speculation: Add force option to GDS mitigation

The Gather Data Sampling (GDS) vulnerability allows malicious software
to infer stale data previously stored in vector registers. This may
include sensitive data such as cryptographic keys. GDS is mitigated in
microcode, and systems with up-to-date microcode are protected by
default. However, any affected system that is running with older
microcode will still be vulnerable to GDS attacks.

Since the gather instructions used by the attacker are part of the
AVX2 and AVX512 extensions, disabling these extensions prevents gather
instructions from being executed, thereby mitigating the system from
GDS. Disabling AVX2 is sufficient, but we don't have the granularity
to do this. The XCR0[2] disables AVX, with no option to just disable
AVX2.

Add a kernel parameter gather_data_sampling=force that will enable the
microcode mitigation if available, otherwise it will disable AVX on
affected systems.

This option will be ignored if cmdline mitigations=off.

This is a *big* hammer.  It is known to break buggy userspace that
uses incomplete, buggy AVX enumeration.  Unfortunately, such userspace
does exist in the wild:

	https://www.mail-archive.com/bug-coreutils@gnu.org/msg33046.html

[ dhansen: add some more ominous warnings about disabling AVX ]

Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>

2 files changed, 21 insertions, 5 deletions

diff --git a/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst b/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst
index 74dab6af7fe148..40b7a626001066 100644
--- a/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst
+++ b/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst
@@ -60,14 +60,21 @@ bits:
  ================================   ===   ============================
 
 GDS can also be mitigated on systems that don't have updated microcode by
-disabling AVX. This can be done by setting "clearcpuid=avx" on the kernel
-command-line.
+disabling AVX. This can be done by setting gather_data_sampling="force" or
+"clearcpuid=avx" on the kernel command-line.
+
+If used, these options will disable AVX use by turning on XSAVE YMM support.
+However, the processor will still enumerate AVX support.  Userspace that
+does not follow proper AVX enumeration to check both AVX *and* XSAVE YMM
+support will break.
 
 Mitigation control on the kernel command line
 ---------------------------------------------
 The mitigation can be disabled by setting "gather_data_sampling=off" or
-"mitigations=off" on the kernel command line. Not specifying either will
-default to the mitigation being enabled.
+"mitigations=off" on the kernel command line. Not specifying either will default
+to the mitigation being enabled. Specifying "gather_data_sampling=force" will
+use the microcode mitigation when available or disable AVX on affected systems
+where the microcode hasn't been updated to include the mitigation.
 
 GDS System Information
 ------------------------
@@ -83,6 +90,9 @@ The possible values contained in this file are:
  Vulnerable                     Processor vulnerable and mitigation disabled.
  Vulnerable: No microcode       Processor vulnerable and microcode is missing
                                 mitigation.
+ Mitigation: AVX disabled,
+ no microcode                   Processor is vulnerable and microcode is missing
+                                mitigation. AVX disabled as mitigation.
  Mitigation: Microcode          Processor is vulnerable and mitigation is in
                                 effect.
  Mitigation: Microcode (locked) Processor is vulnerable and mitigation is in
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c21d42140d6b0e..816b966bed0f58 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1633,7 +1633,13 @@
 
 			This issue is mitigated by default in updated microcode.
 			The mitigation may have a performance impact but can be
-			disabled.
+			disabled. On systems without the microcode mitigation
+			disabling AVX serves as a mitigation.
+
+			force:	Disable AVX to mitigate systems without
+				microcode mitigation. No effect if the microcode
+				mitigation is present. Known to cause crashes in
+				userspace with buggy AVX enumeration.
 
 			off:	Disable GDS mitigation.