Re: [PATCH 0/7] Enable LASS (Linear Address space Separation)

[Sohil Mehta]

Yian,

I added a few missing lists pertaining to KVM, MM and documentation 
since these patches impact them.

In future, scripts/get_maintainer.pl can help you with generating the 
relevant maintainers and lists.

On 1/9/2023 9:51 PM, Yian Chen wrote:
> LASS (Linear Address Space Separation) is a security
> extension that prevents speculative address accesses across
> user/kernel mode. The LASS details have been published in
> Chapter 11 in
> https://cdrdv2.intel.com/v1/dl/getContent/671368
>
> LASS works in 64-bit mode only and partitions the 64-bit
> virtual address space into two halves:
>      1. Lower half (LA[63]=0) --> user space
>      2. Upper half (LA[63]=1) --> kernel space
> When LASS is enabled, a general protection #GP(0) fault will
> be generated if software accesses the address from the half in
> which it resides to another half, e.g., either from user space
> to upper half, or from kernel space to lower half. This
> protection applies to data access, code execution, cache line
> flushing instructions.
>
> Almost all kernel accesses are to the upper half of the virtual
> address space. However, there are valid reasons for kernel to
> access the lower half. For these cases,  kernel can temporarily
> suspend the enforcement of LASS by disabling SMAP (Supervisor
> Mode Access Prevention).
>
> Kernel access to copy data to/from user addresses already
> disables SMAP using the stac()/clac() functions. New functions
> low_addr_access_begin()/low_addr_access_end() are added to
> also disable/enable SMAP around other code that legitimately
> needs to access the lower half of the virtual address space.
>
> User space cannot use any kernel address while LASS is
> enabled. Less fortunately, legacy vsyscall functions used
> by old version of glibc are located in the address range
> 0xffffffffff600000-0xffffffffff601000 and emulated in kernel.
> Therefore, to comply with LASS policy, the legacy vsyscall is
> disabled by default. I am looking for input from Andy and
> others if this approach is acceptable.
>
> This patch set by default enforces LASS when the platform
> supports it. It can be disabled via the command line parameter
> "clearcpuid" or by setting "vsyscall=emulate/xonly".
>
> As of now there is no publicly available CPU supporting LASS.
> The first one to support LASS is Sierra Forest line. The Intel
> Simics® Simulator was used as software development and testing
> vehicle for this patch set.
>
> Paul Lai (1):
>    x86/kvm: Expose LASS feature to VM guest
>
> Yian Chen (6):
>    x86/cpu: Enumerate LASS CPUID and CR4 bits
>    x86: Add CONFIG option X86_LASS
>    x86/cpu: Disable kernel LASS when patching kernel alternatives
>    x86/vsyscall: Setup vsyscall to compromise LASS protection
>    x86/cpu: Enable LASS (Linear Address Space Separation)
>    x86/cpu: Set LASS as pinning sensitive CR4 bit

It's usually good practice to include a base-commit to make it easier to 
apply these patches.

>   .../admin-guide/kernel-parameters.txt         | 12 +++++++----
>   arch/x86/Kconfig                              | 10 +++++++++
>   arch/x86/entry/vsyscall/vsyscall_64.c         | 14 +++++++++++++
>   arch/x86/include/asm/cpufeatures.h            |  1 +
>   arch/x86/include/asm/disabled-features.h      |  8 ++++++-
>   arch/x86/include/asm/kvm_host.h               |  3 ++-
>   arch/x86/include/asm/smap.h                   | 13 ++++++++++++
>   arch/x86/include/uapi/asm/processor-flags.h   |  2 ++
>   arch/x86/kernel/Makefile                      |  2 ++
>   arch/x86/kernel/alternative.c                 | 21 +++++++++++++++++--
>   arch/x86/kernel/cpu/common.c                  | 20 +++++++++++++++++-
>   arch/x86/kvm/cpuid.c                          |  2 +-
>   tools/arch/x86/include/asm/cpufeatures.h      |  1 +
>   .../arch/x86/include/asm/disabled-features.h  |  8 ++++++-
>   tools/objtool/arch/x86/special.c              |  2 ++
>   15 files changed, 108 insertions(+), 11 deletions(-)