Re: [PATCH v2 03/20] x86/mm: temporary mm struct

[Borislav Petkov]

> Subject: Re: [PATCH v2 03/20] x86/mm: temporary mm struct

Subject needs a verb: "Add a temporary... "

On Mon, Jan 28, 2019 at 04:34:05PM -0800, Rick Edgecombe wrote:
> From: Andy Lutomirski <luto@xxxxxxxxxx>
> 
> Sometimes we want to set a temporary page-table entries (PTEs) in one of

s/a //

Also, drop the "we" and make it impartial and passive:

 "Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
  instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
  to do frotz", as if you are giving orders to the codebase to change
  its behaviour."

> 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

exploding or exploiting? Or exposing? :)

> 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.

Those belong in the code comments below, explaining what it is going to
be used for.

> 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: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Tested-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> ---
>  arch/x86/include/asm/mmu_context.h | 32 ++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 19d18fae6ec6..cd0c29e494a6 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -356,4 +356,36 @@ static inline unsigned long __get_current_cr3_fast(void)
>  	return cr3;
>  }
>  
> +typedef struct {

Why does it have to be a typedef?

That prev.prev below looks unnecessary, instead of just using prev.

> +	struct mm_struct *prev;

Why "prev"?

> +} temporary_mm_state_t;

That's kinda long - it is longer than the function name below.
temp_mm_state_t not enough?

> +
> +/*
> + * Using a temporary mm allows to set temporary mappings that are not accessible
> + * by other cores. Such mappings are needed to perform sensitive memory writes
> + * that override the kernel memory protections (e.g., W^X), without exposing the
> + * temporary page-table mappings that are required for these write operations to
> + * other cores.
> + *
> + * Context: The temporary mm needs to be used exclusively by a single core. To
> + *          harden security IRQs must be disabled while the temporary mm is
			      ^
			      ,

> + *          loaded, thereby preventing interrupt handler bugs from override the

s/override/overriding/

> + *          kernel memory protection.
> + */
> +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
> 

-- 
Regards/Gruss,
    Boris.

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