linux-kernel - Re: [PATCH 4/5] efi/x86: Remove extra headroom for setup block

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Message-ID: <db83f5a1-b827-2a31-0ca9-a04df8257324@nextfour.com>
Date:   Mon, 2 Mar 2020 04:21:30 +0000
From:   Mika Penttilä <mika.penttila@...tfour.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>
CC:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] efi/x86: Remove extra headroom for setup block



On 2.3.2020 1.05, Arvind Sankar wrote:
> commit 223e3ee56f77 ("efi/x86: add headroom to decompressor BSS to
> account for setup block") added headroom to the PE image to account for
> the setup block, which wasn't used for the decompression buffer.
>
> Now that we decompress from the start of the image, this is no longer
> required.
>
> Add a check to make sure that the head section of the compressed kernel
> won't overwrite itself while relocating. This is only for
> future-proofing as with current limits on the setup and the actual size
> of the head section, this can never happen.
>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>

To make clear, the kernel (head_32.s and head_64.s) still relocates
itself to the end of the buffer and does in-place decompression. So this
is just to make init sz smaller.


> ---
>  arch/x86/boot/tools/build.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/tools/build.c b/arch/x86/boot/tools/build.c
> index 90d403dfec80..3d03ad753ed5 100644
> --- a/arch/x86/boot/tools/build.c
> +++ b/arch/x86/boot/tools/build.c
> @@ -65,6 +65,8 @@ unsigned long efi_pe_entry;
>  unsigned long efi32_pe_entry;
>  unsigned long kernel_info;
>  unsigned long startup_64;
> +unsigned long _ehead;
> +unsigned long _end;
>  
>  /*----------------------------------------------------------------------*/
>  
> @@ -232,7 +234,7 @@ static void update_pecoff_text(unsigned int text_start, unsigned int file_sz,
>  {
>  	unsigned int pe_header;
>  	unsigned int text_sz = file_sz - text_start;
> -	unsigned int bss_sz = init_sz + text_start - file_sz;
> +	unsigned int bss_sz = init_sz - file_sz;
>  
>  	pe_header = get_unaligned_le32(&buf[0x3c]);
>  
> @@ -259,7 +261,7 @@ static void update_pecoff_text(unsigned int text_start, unsigned int file_sz,
>  	put_unaligned_le32(file_sz - 512 + bss_sz, &buf[pe_header + 0x1c]);
>  
>  	/* Size of image */
> -	put_unaligned_le32(init_sz + text_start, &buf[pe_header + 0x50]);
> +	put_unaligned_le32(init_sz, &buf[pe_header + 0x50]);
>  
>  	/*
>  	 * Address of entry point for PE/COFF executable
> @@ -360,6 +362,8 @@ static void parse_zoffset(char *fname)
>  		PARSE_ZOFS(p, efi32_pe_entry);
>  		PARSE_ZOFS(p, kernel_info);
>  		PARSE_ZOFS(p, startup_64);
> +		PARSE_ZOFS(p, _ehead);
> +		PARSE_ZOFS(p, _end);
>  
>  		p = strchr(p, '\n');
>  		while (p && (*p == '\r' || *p == '\n'))
> @@ -444,6 +448,26 @@ int main(int argc, char ** argv)
>  	put_unaligned_le32(sys_size, &buf[0x1f4]);
>  
>  	init_sz = get_unaligned_le32(&buf[0x260]);
> +#ifdef CONFIG_EFI_STUB
> +	/*
> +	 * The decompression buffer will start at ImageBase. When relocating
> +	 * the compressed kernel to its end, we must ensure that the head
> +	 * section does not get overwritten.  The head section occupies
> +	 * [i, i + _ehead), and the destination is [init_sz - _end, init_sz).
> +	 *
> +	 * At present these should never overlap, because i is at most 32k
> +	 * because of SETUP_SECT_MAX, _ehead is less than 1k, and the
> +	 * calculation of INIT_SIZE in boot/header.S ensures that
> +	 * init_sz - _end is at least 64k.
> +	 *
> +	 * For future-proofing, increase init_sz if necessary.
> +	 */
> +
> +	if (init_sz - _end < i + _ehead) {
> +		init_sz = (i + _ehead + _end + 4095) & ~4095;
> +		put_unaligned_le32(init_sz, &buf[0x260]);
> +	}
> +#endif
>  	update_pecoff_text(setup_sectors * 512, i + (sys_size * 16), init_sz);
>  
>  	efi_stub_entry_update();


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