[PATCH v2] x86/lam: Disable ADDRESS_MASKING in most cases

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

Intel feature Linear Address Masking (LAM) has a weakness related to
transient execution as described in the SLAM paper[1]. Unless Linear
Address Space Separation (LASS) is enabled this weakness may be
exploitable.

Until kernel adds support for LASS[2], only allow LAM for COMPILE_TEST,
or when speculation mitigations have been disabled at compile time,
otherwise keep LAM disabled.

There are no processors in market that support LAM yet, so currently
nobody is affected by this issue.

[1] SLAM: https://download.vusec.net/papers/slam_sp24.pdf
[2] LASS: https://lore.kernel.org/lkml/20230609183632.48706-1-alexander.shishkin@linux.intel.com/

Cc: stable@vger.kernel.org # v6.4+
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
v2:
- Updated commit message to reflect that currently there is no exposure
  of this issue.
- Added review tags.

 arch/x86/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1566748f16c4..794517df8068 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2270,6 +2270,7 @@ config RANDOMIZE_MEMORY_PHYSICAL_PADDING
 config ADDRESS_MASKING
 	bool "Linear Address Masking support"
 	depends on X86_64
+	depends on COMPILE_TEST || !SPECULATION_MITIGATIONS # wait for LASS
 	help
 	  Linear Address Masking (LAM) modifies the checking that is applied
 	  to 64-bit linear addresses, allowing software to use of the

base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
-- 
2.34.1