Date:   Mon, 9 Nov 2020 15:00:44 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        thomas.lendacky@....com, jroedel@...e.de
Subject: Re: [RFC][PATCH 00/24] x86/pti: Defer CR3 switch to C code


Sorry but it looks like email addresses are messed up in my emails. Our email
server has a new security "feature" which has the good idea to change external
email addresses.

I will resend the patches with the correct addresses once I've found
how to prevent this mess.

alex.

On 11/9/20 12:22 PM, Alexandre Chartre wrote:
> With Page Table Isolation (PTI), syscalls as well as interrupts and
> exceptions occurring in userspace enter the kernel with a user
> page-table. The kernel entry code will then switch the page-table
> from the user page-table to the kernel page-table by updating the
> CR3 control register. This CR3 switch is currently done early in
> the kernel entry sequence using assembly code.
> 
> This RFC proposes to defer the PTI CR3 switch until we reach C code.
> The benefit is that this simplifies the assembly entry code, and make
> the PTI CR3 switch code easier to understand. This also paves the way
> for further possible projects such an easier integration of Address
> Space Isolation (ASI), or the possibilily to execute some selected
> syscall or interrupt handlers without switching to the kernel page-table
> (and thus avoid the PTI page-table switch overhead).
> 
> Deferring CR3 switch to C code means that we need to run more of the
> kernel entry code with the user page-table. To do so, we need to:
> 
>   - map more syscall, interrupt and exception entry code into the user
>     page-table (map all noinstr code);
> 
>   - map additional data used in the entry code (such as stack canary);
> 
>   - run more entry code on the trampoline stack (which is mapped both
>     in the kernel and in the user page-table) until we switch to the
>     kernel page-table and then switch to the kernel stack;
> 
>   - have a per-task trampoline stack instead of a per-cpu trampoline
>     stack, so the task can be scheduled out while it hasn't switched
>     to the kernel stack.
> 
> Note that, for now, the CR3 switch can only be pushed as far as interrupts
> remain disabled in the entry code. This is because the CR3 switch is done
> based on the privilege level from the CS register from the interrupt frame.
> I plan to fix this but that's some extra complication (need to track if the
> user page-table is used or not).
> 
> The proposed patchset is in RFC state to get early feedback about this
> proposal.
> 
> The code survives running a kernel build and LTP. Note that changes are
> only for 64-bit at the moment, I haven't looked at 32-bit yet but I will
> definitively check it.
> 
> Code is based on v5.10-rc3.
> 
> Thanks,
> 
> alex.
> 
> -----
> 
> Alexandre Chartre (24):
>    x86/syscall: Add wrapper for invoking syscall function
>    x86/entry: Update asm_call_on_stack to support more function arguments
>    x86/entry: Consolidate IST entry from userspace
>    x86/sev-es: Define a setup stack function for the VC idtentry
>    x86/entry: Implement ret_from_fork body with C code
>    x86/pti: Provide C variants of PTI switch CR3 macros
>    x86/entry: Fill ESPFIX stack using C code
>    x86/entry: Add C version of SWAPGS and SWAPGS_UNSAFE_STACK
>    x86/entry: Add C version of paranoid_entry/exit
>    x86/pti: Introduce per-task PTI trampoline stack
>    x86/pti: Function to clone page-table entries from a specified mm
>    x86/pti: Function to map per-cpu page-table entry
>    x86/pti: Extend PTI user mappings
>    x86/pti: Use PTI stack instead of trampoline stack
>    x86/pti: Execute syscall functions on the kernel stack
>    x86/pti: Execute IDT handlers on the kernel stack
>    x86/pti: Execute IDT handlers with error code on the kernel stack
>    x86/pti: Execute system vector handlers on the kernel stack
>    x86/pti: Execute page fault handler on the kernel stack
>    x86/pti: Execute NMI handler on the kernel stack
>    x86/entry: Disable stack-protector for IST entry C handlers
>    x86/entry: Defer paranoid entry/exit to C code
>    x86/entry: Remove paranoid_entry and paranoid_exit
>    x86/pti: Defer CR3 switch to C code for non-IST and syscall entries
> 
>   arch/x86/entry/common.c               | 259 ++++++++++++-
>   arch/x86/entry/entry_64.S             | 513 ++++++++------------------
>   arch/x86/entry/entry_64_compat.S      |  22 --
>   arch/x86/include/asm/entry-common.h   | 108 ++++++
>   arch/x86/include/asm/idtentry.h       | 153 +++++++-
>   arch/x86/include/asm/irq_stack.h      |  11 +
>   arch/x86/include/asm/page_64_types.h  |  36 +-
>   arch/x86/include/asm/paravirt.h       |  15 +
>   arch/x86/include/asm/paravirt_types.h |  17 +-
>   arch/x86/include/asm/processor.h      |   3 +
>   arch/x86/include/asm/pti.h            |  18 +
>   arch/x86/include/asm/switch_to.h      |   7 +-
>   arch/x86/include/asm/traps.h          |   2 +-
>   arch/x86/kernel/cpu/mce/core.c        |   7 +-
>   arch/x86/kernel/espfix_64.c           |  41 ++
>   arch/x86/kernel/nmi.c                 |  34 +-
>   arch/x86/kernel/sev-es.c              |  52 +++
>   arch/x86/kernel/traps.c               |  61 +--
>   arch/x86/mm/fault.c                   |  11 +-
>   arch/x86/mm/pti.c                     |  71 ++--
>   kernel/fork.c                         |  22 ++
>   21 files changed, 1002 insertions(+), 461 deletions(-)
> 

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