Re: [Xen-devel] [PATCH v1 0/6] x86: Enable Supervisor Mode Execution Prevention (SMAP)

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

the title is incorrect: "Supervisor Mode Execution Prevention (SMAP)"

> From: Feng Wu
> Sent: Tuesday, April 15, 2014 8:58 PM
> 
> Supervisor Mode Access Prevention (SMAP) is a new security
> feature disclosed by Intel, please refer to the following
> document:
> 
> http://software.intel.com/sites/default/files/319433-014.pdf
> 
> Every access to a linear address is either a supervisor-mode
> access or a user-mode access. All accesses performed while the
> current privilege level (CPL) is less than 3 are supervisor-mode
> accesses. If CPL = 3, accesses are generally user-mode accesses.
> However, some operations implicitly access system data structures,
> and the resulting accesses to those data structures are supervisor-mode
> accesses regardless of CPL. Examples of such implicit supervisor
> accesses include the following: accesses to the global descriptor
> table (GDT) or local descriptor table (LDT) to load a segment descriptor;
> accesses to the interrupt descriptor table (IDT) when delivering an
> interrupt or exception; and accesses to the task-state segment (TSS) as
> part of a task switch or change of CPL.
> 
> If CR4.SMAP = 1, supervisor-mode data accesses are not allowed
> to linear addresses that are accessible in user mode. If CPL < 3,
> SMAP protections are disabled if EFLAGS.AC = 1. If CPL = 3, SMAP
> applies to all supervisor-mode data accesses (these are implicit
> supervisor accesses) regardless of the value of EFLAGS.AC.
> 
> Version 1:
>   * Add two macros for STAC/CLAC instructions
>   * Temporary disable SMAP to legally access user pages in kernel mode
>   * Enable Supervisor Mode Execution Prevention (SMAP) for Xen itself
>   * Add SMAP support to HVM guest
>   * Disable SMAP feature when guest is in non-paging mode
> 
> Since Linux kernel has already supported SMAP, I tested this patch both
> in EPT and shadow mode with Linux guest, they work well. And I also
> tested the failure case, in which, I triggered an SMAP violation in
> the guest kernel and Linux successfully received the related page
> fault.
> 
> Feng Wu (6):
>   x86: Add support for STAC/CLAC instructions
>   x86: Temporary disable SMAP to legally access user pages in kernel
>     mode
>   x86: Enable Supervisor Mode Execution Prevention (SMAP) for Xen
>   x86/hvm: Add SMAP support to HVM guest
>   VMX: Disable SMAP feature when guest is in non-paging mode
>   x86/tools: Expose SMAP to HVM guests
> 
>  tools/libxc/xc_cpufeature.h            |  1 +
>  tools/libxc/xc_cpuid_x86.c             |  1 +
>  xen/arch/x86/clear_page.S              |  3 ++
>  xen/arch/x86/domain_build.c            | 16 ++++++++
>  xen/arch/x86/hvm/hvm.c                 |  3 ++
>  xen/arch/x86/hvm/vmx/vmx.c             |  6 +--
>  xen/arch/x86/mm/guest_walk.c           | 27 +++++++++++--
>  xen/arch/x86/setup.c                   |  9 +++++
>  xen/arch/x86/traps.c                   | 34 +++++++++++++----
>  xen/arch/x86/usercopy.c                |  6 +++
>  xen/arch/x86/x86_64/asm-offsets.c      |  1 +
>  xen/arch/x86/x86_64/compat/entry.S     |  2 +
>  xen/arch/x86/x86_64/entry.S            |  4 ++
>  xen/include/asm-x86/cpufeature.h       |  1 +
>  xen/include/asm-x86/domain.h           |  6 ++-
>  xen/include/asm-x86/hvm/hvm.h          | 13 +++++++
>  xen/include/asm-x86/uaccess.h          |  4 ++
>  xen/include/asm-x86/x86_64/asm_defns.h | 70
> ++++++++++++++++++++++++++++++++++
>  xen/include/asm-x86/x86_64/system.h    |  2 +
>  19 files changed, 192 insertions(+), 17 deletions(-)
> 
> --
> 1.8.3.1
> 
> 
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