linux-hardening - Re: [PATCH 08/16] x86/boot: Remove mapping from page fault handler

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Date:   Wed, 19 Oct 2022 09:20:34 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Evgeniy Baskov <baskov@...ras.ru>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        lvc-project@...uxtesting.org, x86@...nel.org,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 08/16] x86/boot: Remove mapping from page fault handler

On Tue, 6 Sept 2022 at 12:41, Evgeniy Baskov <baskov@...ras.ru> wrote:
>
> After every implicit mapping is removed, this code is no longer needed.
>
> Remove memory mapping from page fault handler to ensure that there are
> no hidden invalid memory accesses.
>
> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>

I don't grok this 100% but to me, it seems not having to rely on a
page fault handler to ensure that the 1:1 mapping has sufficient
coverage is a win so

Acked-by: Ard Biesheuvel <ardb@...nel.org>

> ---
>  arch/x86/boot/compressed/ident_map_64.c | 26 ++++++++++---------------
>  1 file changed, 10 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index 880e08293023..c20cd31e665f 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -385,27 +385,21 @@ void do_boot_page_fault(struct pt_regs *regs, unsigned long error_code)
>  {
>         unsigned long address = native_read_cr2();
>         unsigned long end;
> -       bool ghcb_fault;
> +       char *msg;
>
> -       ghcb_fault = sev_es_check_ghcb_fault(address);
> +       if (sev_es_check_ghcb_fault(address))
> +               msg = "Page-fault on GHCB page:";
> +       else
> +               msg = "Unexpected page-fault:";
>
>         address   &= PMD_MASK;
>         end        = address + PMD_SIZE;
>
>         /*
> -        * Check for unexpected error codes. Unexpected are:
> -        *      - Faults on present pages
> -        *      - User faults
> -        *      - Reserved bits set
> -        */
> -       if (error_code & (X86_PF_PROT | X86_PF_USER | X86_PF_RSVD))
> -               do_pf_error("Unexpected page-fault:", error_code, address, regs->ip);
> -       else if (ghcb_fault)
> -               do_pf_error("Page-fault on GHCB page:", error_code, address, regs->ip);
> -
> -       /*
> -        * Error code is sane - now identity map the 2M region around
> -        * the faulting address.
> +        * Since all memory allocations are made explicit
> +        * now, every page fault at this stage is an
> +        * error and the error handler is there only
> +        * for debug purposes.
>          */
> -       kernel_add_identity_map(address, end, MAP_WRITE);
> +       do_pf_error(msg, error_code, address, regs->ip);
>  }
> --
> 2.35.1
>