linux-kernel - Re: [PATCH 13/14] arm64: kexec_file: add Image format support

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Date:   Thu, 24 Aug 2017 18:23:37 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     catalin.marinas@....com, will.deacon@....com,
        bauerman@...ux.vnet.ibm.com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, akpm@...ux-foundation.org, mpe@...erman.id.au,
        dyoung@...hat.com, bhe@...hat.com, arnd@...db.de,
        ard.biesheuvel@...aro.org, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 13/14] arm64: kexec_file: add Image format support

On Thu, Aug 24, 2017 at 05:18:10PM +0900, AKASHI Takahiro wrote:
> The "Image" binary will be loaded at the offset of TEXT_OFFSET from
> the start of system memory. TEXT_OFFSET is basically determined from
> the header of the image.

What's the policy for the binary types kexec_file_load() will load, and
how are these identified? AFAICT, there are no flags, so it looks like
we're just checking the magic and hoping.

> Regarding kernel verification, it will be done through
> verify_pefile_signature() as arm64's "Image" binary can be seen as
> in PE format. This approach is consistent with x86 implementation.

This will not work for kernels built without CONFIG_EFI, where we don't
have a PE header.

What happens in that case?

[...]

> +/**
> + * arm64_header_check_msb - Helper to check the arm64 image header.
> + *
> + * Returns non-zero if the image was built as big endian.
> + */
> +
> +static inline int arm64_header_check_msb(const struct arm64_image_header *h)
> +{
> +	if (!h)
> +		return 0;
> +
> +	return !!(h->flags[7] & arm64_image_flag_7_be);
> +}

What are we going to use this for?

In kernel, we use the term "BE" rather than "MSB", and it's unfortunate
to have code with varying naming conventions.

[...]

> +static void *image_load(struct kimage *image, char *kernel,
> +			    unsigned long kernel_len, char *initrd,
> +			    unsigned long initrd_len, char *cmdline,
> +			    unsigned long cmdline_len)
> +{
> +	struct kexec_buf kbuf;
> +	struct arm64_image_header *h = (struct arm64_image_header *)kernel;
> +	unsigned long text_offset, kernel_load_addr;
> +	int ret;
> +
> +	/* Create elf core header segment */
> +	ret = load_crashdump_segments(image);
> +	if (ret)
> +		goto out;
> +
> +	/* Load the kernel */
> +	kbuf.image = image;
> +	if (image->type == KEXEC_TYPE_CRASH) {
> +		kbuf.buf_min = crashk_res.start;
> +		kbuf.buf_max = crashk_res.end + 1;
> +	} else {
> +		kbuf.buf_min = 0;
> +		kbuf.buf_max = ULONG_MAX;
> +	}
> +	kbuf.top_down = 0;
> +
> +	kbuf.buffer = kernel;
> +	kbuf.bufsz = kernel_len;
> +	if (h->image_size) {
> +		kbuf.memsz = le64_to_cpu(h->image_size);
> +		text_offset = le64_to_cpu(h->text_offset);
> +	} else {
> +		/* v3.16 or older */
> +		kbuf.memsz = kbuf.bufsz; /* NOTE: not including BSS */

Why bother supporting < 3.16 kernels?

They predate regulate kexec, we know we don't have enough information to
boot such kernels reliably, and arguably attempting to load one would
indicate some kind of rollback attack.

Thanks,
Mark.