[PATCHv7 00/16] x86: Enable Linear Address Space Separation support

Kirill A. Shutemov
2025-06-25T12:51:26Z

Linear Address Space Separation (LASS) is a security feature that intends to
prevent malicious virtual address space accesses across user/kernel mode.

Such mode based access protection already exists today with paging and features
such as SMEP and SMAP. However, to enforce these protections, the processor
must traverse the paging structures in memory. Malicious software can use
timing information resulting from this traversal to determine details about the
paging structures, and these details may also be used to determine the layout
of the kernel memory.

The LASS mechanism provides the same mode-based protections as paging but
without traversing the paging structures. Because the protections enforced by
LASS are applied before paging, software will not be able to derive
paging-based timing information from the various caching structures such as the
TLBs, mid-level caches, page walker, data caches, etc. LASS can avoid probing
using double page faults, TLB flush and reload, and SW prefetch instructions.
See [2], [3] and [4] for some research on the related attack vectors.

Had it been available, LASS alone would have mitigated Meltdown. (Hindsight is
20/20 :)

In addition, LASS prevents an attack vector described in a Spectre LAM (SLAM)
whitepaper [7].

LASS enforcement relies on the typical kernel implementation to divide the
64-bit virtual address space into two halves:
Addr[63]=0 -> User address space
Addr[63]=1 -> Kernel address space
Any data access or code execution across address spaces typically results in a
#GP fault.

Kernel accesses usually only happen to the kernel address space. However, there
are valid reasons for kernel to access memory in the user half. For these cases
(such as text poking and EFI runtime accesses), the kernel can temporarily
suspend the enforcement of LASS by toggling SMAP (Supervisor Mode Access
Prevention) using the stac()/clac() instructions and in one instance a downright
disabling LASS for an EFI runtime call.

User space cannot access any kernel address while LASS is enabled.
Unfortunately, legacy vsyscall functions are located in the address range
0xffffffffff600000 - 0xffffffffff601000 and emulated in kernel. To avoid
breaking user applications when LASS is enabled, extend the vsyscall emulation
in execute (XONLY) mode to the #GP fault handler.

In contrast, the vsyscall EMULATE mode is deprecated and not expected to be
used by anyone. Supporting EMULATE mode with LASS would need complex
instruction decoding in the #GP fault handler and is probably not worth the
hassle. Disable LASS in this rare case when someone absolutely needs and
enables vsyscall=emulate via the command line.

Changes from v6[10]:
- Rewrok #SS handler to work properly on FRED;
- Do not require X86_PF_INSTR to emulate vsyscall;
- Move lass_clac()/stac() definition to the patch where they are used;
- Rename lass_clac/stac() to lass_disable/enable_enforcement();
- Fix several build issues around inline memcpy and memset;
- Fix sparse warning;
- Adjust comments and commit messages;
- Drop "x86/efi: Move runtime service initialization to arch/x86" patch
as it got applied;

Changes from v5[9]:
- Report LASS violation as NULL pointer dereference if the address is in the
first page frame;
- Provide helpful error message on #SS due to LASS violation;
- Fold patch for vsyscall=emulate documentation into patch
that disables LASS with vsyscall=emulate;
- Rewrite __inline_memeset() and __inline_memcpy();
- Adjust comments and commit messages;

Changes from v4[8]:
- Added PeterZ's Originally-by and SoB to 2/16
- Added lass_clac()/lass_stac() to differentiate from SMAP necessitated
clac()/stac() and to be NOPs on CPUs that don't support LASS
- Moved LASS enabling patch to the end to avoid rendering machines
unbootable between until the patch that disables LASS around EFI
initialization
- Reverted Pawan's LAM disabling commit

Changes from v3[6]:
- Made LAM dependent on LASS
- Moved EFI runtime initialization to x86 side of things
- Suspended LASS validation around EFI set_virtual_address_map call
- Added a message for the case of kernel side LASS violation
- Moved inline memset/memcpy versions to the common string.h

Changes from v2[5]:
- Added myself to the SoB chain

Changes from v1[1]:
- Emulate vsyscall violations in execute mode in the #GP fault handler
- Use inline memcpy and memset while patching alternatives
- Remove CONFIG_X86_LASS
- Make LASS depend on SMAP
- Dropped the minimal KVM enabling patch

[1] https://lore.kernel.org/lkml/20230110055204.3227669-1-yian.chen@intel.com/
[2] “Practical Timing Side Channel Attacks against Kernel Space ASLR”,
https://www.ieee-security.org/TC/SP2013/papers/4977a191.pdf
[3] “Prefetch Side-Channel Attacks: Bypassing SMAP and Kernel ASLR”, http://doi.acm.org/10.1145/2976749.2978356
[4] “Harmful prefetch on Intel”, https://ioactive.com/harmful-prefetch-on-intel/ (H/T Anders)
[5] https://lore.kernel.org/all/20230530114247.21821-1-alexander.shishkin@linux.intel.com/
[6] https://lore.kernel.org/all/20230609183632.48706-1-alexander.shishkin@linux.intel.com/
[7] https://download.vusec.net/papers/slam_sp24.pdf
[8] https://lore.kernel.org/all/20240710160655.3402786-1-alexander.shishkin@linux.intel.com/
[9] https://lore.kernel.org/all/20241028160917.1380714-1-alexander.shishkin@linux.intel.com
[10] https://lore.kernel.org/all/20250620135325.3300848-1-kirill.shutemov@linux.intel.com/

Alexander Shishkin (4):
x86/cpu: Defer CR pinning setup until after EFI initialization
efi: Disable LASS around set_virtual_address_map() EFI call
x86/traps: Communicate a LASS violation in #GP message
x86/cpu: Make LAM depend on LASS

Kirill A. Shutemov (4):
x86/asm: Introduce inline memcpy and memset
x86/vsyscall: Do not require X86_PF_INSTR to emulate vsyscall
x86/traps: Handle LASS thrown #SS
x86: Re-enable Linear Address Masking

Sohil Mehta (7):
x86/cpu: Enumerate the LASS feature bits
x86/alternatives: Disable LASS when patching kernel alternatives
x86/vsyscall: Reorganize the #PF emulation code
x86/traps: Consolidate user fixups in exc_general_protection()
x86/vsyscall: Add vsyscall emulation for #GP
x86/vsyscall: Disable LASS if vsyscall mode is set to EMULATE
x86/cpu: Enable LASS during CPU initialization

Yian Chen (1):
x86/cpu: Set LASS CR4 bit as pinning sensitive

.../admin-guide/kernel-parameters.txt | 4 +-
arch/x86/Kconfig | 1 -
arch/x86/Kconfig.cpufeatures | 4 +
arch/x86/entry/vsyscall/vsyscall_64.c | 69 +++++++++++------
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/smap.h | 27 ++++++-
arch/x86/include/asm/string.h | 46 ++++++++++++
arch/x86/include/asm/uaccess_64.h | 38 +++-------
arch/x86/include/asm/vsyscall.h | 14 +++-
arch/x86/include/uapi/asm/processor-flags.h | 2 +
arch/x86/kernel/alternative.c | 14 +++-
arch/x86/kernel/cpu/common.c | 21 ++++--
arch/x86/kernel/cpu/cpuid-deps.c | 2 +
arch/x86/kernel/traps.c | 75 +++++++++++++++----
arch/x86/kernel/umip.c | 3 +
arch/x86/lib/clear_page_64.S | 10 ++-
arch/x86/mm/fault.c | 2 +-
arch/x86/platform/efi/efi.c | 15 ++++
tools/arch/x86/include/asm/cpufeatures.h | 1 +
19 files changed, 264 insertions(+), 85 deletions(-)

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2.47.2