linux-kernel - Re: [RFC PATCH 2/6] x86/mm: temporary mm struct

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Date:   Wed, 29 Aug 2018 08:46:35 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 2/6] x86/mm: temporary mm struct

Rik, this is the patch I was referring to.

On Wed, Aug 29, 2018 at 1:11 AM, Nadav Amit <namit@...are.com> wrote:
> From: Andy Lutomirski <luto@...nel.org>
>
> Sometimes we want to set a temporary page-table entries (PTEs) in one of
> the cores, without allowing other cores to use - even speculatively -
> these mappings. There are two benefits for doing so:
>
> (1) Security: if sensitive PTEs are set, temporary mm prevents their use
> in other cores. This hardens the security as it prevents exploding a
> dangling pointer to overwrite sensitive data using the sensitive PTE.
>
> (2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
> remote page-tables.
>
> To do so a temporary mm_struct can be used. Mappings which are private
> for this mm can be set in the userspace part of the address-space.
> During the whole time in which the temporary mm is loaded, interrupts
> must be disabled.
>
> The first use-case for temporary PTEs, which will follow, is for poking
> the kernel text.
>
> [ Commit message was written by Nadav ]
>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
>  arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index eeeb9289c764..96afc8c0cf15 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void)
>         return cr3;
>  }
>
> +typedef struct {
> +       struct mm_struct *prev;
> +} temporary_mm_state_t;
> +
> +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
> +{
> +       temporary_mm_state_t state;
> +
> +       lockdep_assert_irqs_disabled();
> +       state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
> +       switch_mm_irqs_off(NULL, mm, current);
> +       return state;
> +}
> +
> +static inline void unuse_temporary_mm(temporary_mm_state_t prev)
> +{
> +       lockdep_assert_irqs_disabled();
> +       switch_mm_irqs_off(NULL, prev.prev, current);
> +}
> +
>  #endif /* _ASM_X86_MMU_CONTEXT_H */
> --
> 2.17.1
>