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Message-ID: <66857084-fbed-3e9a-ed2c-7167010cbad9@intel.com>
Date: Tue, 10 Jan 2023 11:48:41 -0800
From: Sohil Mehta <sohil.mehta@...el.com>
To: Yian Chen <yian.chen@...el.com>, <linux-kernel@...r.kernel.org>,
<x86@...nel.org>, Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Ravi Shankar <ravi.v.shankar@...el.com>,
"Tony Luck" <tony.luck@...el.com>, Paul Lai <paul.c.lai@...el.com>,
<kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
<linux-mm@...ck.org>
Subject: Re: [PATCH 0/7] Enable LASS (Linear Address space Separation)
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
> SimicsĀ® 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(-)
>
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