linux-kernel - Re: [PATCH v3 1/4] x86/boot/64: Explicitly map boot

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Message-ID: <20201019145115.GB24325@zn.tnic>
Date:   Mon, 19 Oct 2020 16:51:15 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     x86@...nel.org, Joerg Roedel <jroedel@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/4] x86/boot/64: Explicitly map boot_params and
 command line

On Fri, Oct 16, 2020 at 04:04:01PM -0400, Arvind Sankar wrote:
> Commits
> 
>   ca0e22d4f011 ("x86/boot/compressed/64: Always switch to own page table")
>   8570978ea030 ("x86/boot/compressed/64: Don't pre-map memory in KASLR code")
> 
> set up a new page table in the decompressor stub, but without explicit
> mappings for boot_params and the kernel command line, relying on the #PF
> handler instead.
> 
> This is fragile, as boot_params and the command line mappings are
> required for the main kernel. If EARLY_PRINTK and RANDOMIZE_BASE are
> disabled, a QEMU/OVMF boot never accesses the command line in the
> decompressor stub, and so it never gets mapped. The main kernel accesses
> it from the identity mapping if AMD_MEM_ENCRYPT is enabled, and will
> crash.
> 
> Fix this by adding back the explicit mapping of boot_params and the
> command line.
> 
> Note: the changes also removed the explicit mapping of the main kernel,
> with the result that .bss and .brk may not be in the identity mapping,
> but those don't get accessed by the main kernel before it switches to
> its own page tables.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> Reviewed-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/boot/compressed/head_64.S      |  3 +++
>  arch/x86/boot/compressed/ident_map_64.c | 24 +++++++++++++++++++++---
>  2 files changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 1c80f1738fd9..3976b4e92e1b 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -544,6 +544,9 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
>  	pushq	%rsi
>  	call	set_sev_encryption_mask
>  	call	load_stage2_idt
> +	/* Pass boot_params to initialize_identity_maps */
> +	popq	%rdi
> +	pushq	%rdi

Any reason why you're not doing

	movq    (%rsp), %rdi

here instead?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette