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Message-ID: <20190131112948.GE6749@zn.tnic>
Date: Thu, 31 Jan 2019 12:29:48 +0100
From: Borislav Petkov <bp@...en8.de>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
Thomas Gleixner <tglx@...utronix.de>,
Nadav Amit <nadav.amit@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Peter Zijlstra <peterz@...radead.org>, linux_dti@...oud.com,
linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org, akpm@...ux-foundation.org,
kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
will.deacon@....com, ard.biesheuvel@...aro.org,
kristen@...ux.intel.com, deneen.t.dock@...el.com,
Kees Cook <keescook@...omium.org>,
Dave Hansen <dave.hansen@...el.com>, Nadav Amit <namit@...are.com>
Subject: Re: [PATCH v2 03/20] x86/mm: temporary mm struct
> 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@...nel.org>
>
> 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@...omium.org>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
> Tested-by: Masami Hiramatsu <mhiramat@...nel.org>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Nadav Amit <namit@...are.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
> 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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