[kernel-hardening] Re: [PATCH] ARM: mm: add testcases for RODATA

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jan 18, 2017 at 10:53:10PM +0900, Jinbum Park wrote:
> This patch adds testcases for the CONFIG_DEBUG_RODATA option.
> It's similar to x86's testcases.
> It tests read-only mapped data and page-size aligned rodata section.

I note that LKDTM already has a similar test (though it just has a raw
write, and will crash the kernel).

> +	asm volatile(
> +		"0:	str %[zero], [%[rodata_test]]\n"
> +		"	mov %[rslt], %[zero]\n"
> +		"1:\n"
> +		".pushsection .text.fixup,\"ax\"\n"
> +		".align 2\n"
> +		"2:\n"
> +		"b 1b\n"
> +		".popsection\n"
> +		".pushsection __ex_table,\"a\"\n"
> +		".align 3\n"
> +		".long 0b, 2b\n"
> +		".popsection\n"
> +		: [rslt] "=r" (result)
> +		: [zero] "r" (0UL), [rodata_test] "r" (&rodata_test_data)
> +	);

This is the only architecture-specific part of the file.

Rather than duplicating the logic from x86, can't we use generic
infrastructure for this part, and move the existing test into a shared
location?

e.g. could we change to KERNEL_DS and use put_user here?

> +	if (!result) {
> +		pr_err("rodata_test: test data was not read only\n");
> +		return -ENODEV;
> +	}
> +
> +	/* test 3: check the value hasn't changed */
> +	/* If this test fails, we managed to overwrite the data */
> +	if (!rodata_test_data) {
> +		pr_err("rodata_test: Test 3 fails (end data)\n");
> +		return -ENODEV;
> +	}
> +
> +	/* test 4: check if the rodata section is 4Kb aligned */
> +	start = (unsigned long)__start_rodata;
> +	end = (unsigned long)__end_rodata;
> +	if (start & (PAGE_SIZE - 1)) {
> +		pr_err("rodata_test: .rodata is not 4k aligned\n");
> +		return -ENODEV;
> +	}
> +	if (end & (PAGE_SIZE - 1)) {
> +		pr_err("rodata_test: .rodata end is not 4k aligned\n");
> +		return -ENODEV;
> +	}

s/4k/page/ in the prints, if this becomes generic.

Thanks,
Mark.