Message-ID: <d2ed1821-96a2-d7eb-4c1e-4787c7dd467a@c-s.fr>
Date:   Mon, 29 Jul 2019 13:33:29 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Jason Yan <yanaijie@...wei.com>, mpe@...erman.id.au,
        linuxppc-dev@...ts.ozlabs.org, diana.craciun@....com,
        benh@...nel.crashing.org, paulus@...ba.org, npiggin@...il.com,
        keescook@...omium.org, kernel-hardening@...ts.openwall.com
Cc:     linux-kernel@...r.kernel.org, wangkefeng.wang@...wei.com,
        yebin10@...wei.com, thunder.leizhen@...wei.com,
        jingxiangfeng@...wei.com, fanchengyang@...wei.com
Subject: Re: [RFC PATCH 07/10] powerpc/fsl_booke/32: randomize the kernel
 image offset



Le 17/07/2019 à 10:06, Jason Yan a écrit :
> After we have the basic support of relocate the kernel in some
> appropriate place, we can start to randomize the offset now.
> 
> Entropy is derived from the banner and timer, which will change every
> build and boot. This not so much safe so additionally the bootloader may
> pass entropy via the /chosen/kaslr-seed node in device tree.
> 
> We will use the first 512M of the low memory to randomize the kernel
> image. The memory will be split in 64M zones. We will use the lower 8
> bit of the entropy to decide the index of the 64M zone. Then we chose a
> 16K aligned offset inside the 64M zone to put the kernel in.
> 
>      KERNELBASE
> 
>          |-->   64M   <--|
>          |               |
>          +---------------+    +----------------+---------------+
>          |               |....|    |kernel|    |               |
>          +---------------+    +----------------+---------------+
>          |                         |
>          |----->   offset    <-----|
> 
>                                kimage_vaddr
> 
> We also check if we will overlap with some areas like the dtb area, the
> initrd area or the crashkernel area. If we cannot find a proper area,
> kaslr will be disabled and boot from the original kernel.
> 
> Signed-off-by: Jason Yan <yanaijie@...wei.com>
> Cc: Diana Craciun <diana.craciun@....com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Christophe Leroy <christophe.leroy@....fr>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: Kees Cook <keescook@...omium.org>
> ---
>   arch/powerpc/kernel/kaslr_booke.c | 335 +++++++++++++++++++++++++++++-
>   1 file changed, 333 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kaslr_booke.c b/arch/powerpc/kernel/kaslr_booke.c
> index 72d8e9432048..90357f4bd313 100644
> --- a/arch/powerpc/kernel/kaslr_booke.c
> +++ b/arch/powerpc/kernel/kaslr_booke.c
> @@ -22,6 +22,8 @@
>   #include <linux/delay.h>
>   #include <linux/highmem.h>
>   #include <linux/memblock.h>
> +#include <linux/libfdt.h>
> +#include <linux/crash_core.h>
>   #include <asm/pgalloc.h>
>   #include <asm/prom.h>
>   #include <asm/io.h>
> @@ -33,15 +35,342 @@
>   #include <asm/machdep.h>
>   #include <asm/setup.h>
>   #include <asm/paca.h>
> +#include <asm/kdump.h>
>   #include <mm/mmu_decl.h>
> +#include <generated/compile.h>
> +#include <generated/utsrelease.h>
> +
> +#ifdef DEBUG
> +#define DBG(fmt...) printk(KERN_ERR fmt)
> +#else
> +#define DBG(fmt...)
> +#endif
> +
> +struct regions {
> +	unsigned long pa_start;
> +	unsigned long pa_end;
> +	unsigned long kernel_size;
> +	unsigned long dtb_start;
> +	unsigned long dtb_end;
> +	unsigned long initrd_start;
> +	unsigned long initrd_end;
> +	unsigned long crash_start;
> +	unsigned long crash_end;
> +	int reserved_mem;
> +	int reserved_mem_addr_cells;
> +	int reserved_mem_size_cells;
> +};
>   
>   extern int is_second_reloc;
>   
> +/* Simplified build-specific string for starting entropy. */
> +static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
> +		LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
> +static char __initdata early_command_line[COMMAND_LINE_SIZE];
> +
> +static __init void kaslr_get_cmdline(void *fdt)
> +{
> +	const char *cmdline = CONFIG_CMDLINE;
> +	if (!IS_ENABLED(CONFIG_CMDLINE_FORCE)) {
> +		int node;
> +		const u8 *prop;
> +		node = fdt_path_offset(fdt, "/chosen");
> +		if (node < 0)
> +			goto out;
> +
> +		prop = fdt_getprop(fdt, node, "bootargs", NULL);
> +		if (!prop)
> +			goto out;
> +		cmdline = prop;
> +	}
> +out:
> +	strscpy(early_command_line, cmdline, COMMAND_LINE_SIZE);
> +}
> +

Can you explain why we need that and can't use the already existing 
cmdline stuff ?

Christophe

> +static unsigned long __init rotate_xor(unsigned long hash, const void *area,
> +				size_t size)
> +{
> +	size_t i;
> +	unsigned long *ptr = (unsigned long *)area;
> +
> +	for (i = 0; i < size / sizeof(hash); i++) {
> +		/* Rotate by odd number of bits and XOR. */
> +		hash = (hash << ((sizeof(hash) * 8) - 7)) | (hash >> 7);
> +		hash ^= ptr[i];
> +	}
> +
> +	return hash;
> +}
> +
> +/* Attempt to create a simple but unpredictable starting entropy. */
> +static unsigned long __init get_boot_seed(void *fdt)
> +{
> +	unsigned long hash = 0;
> +
> +	hash = rotate_xor(hash, build_str, sizeof(build_str));
> +	hash = rotate_xor(hash, fdt, fdt_totalsize(fdt));
> +
> +	return hash;
> +}
> +
> +static __init u64 get_kaslr_seed(void *fdt)
> +{
> +	int node, len;
> +	fdt64_t *prop;
> +	u64 ret;
> +
> +	node = fdt_path_offset(fdt, "/chosen");
> +	if (node < 0)
> +		return 0;
> +
> +	prop = fdt_getprop_w(fdt, node, "kaslr-seed", &len);
> +	if (!prop || len != sizeof(u64))
> +		return 0;
> +
> +	ret = fdt64_to_cpu(*prop);
> +	*prop = 0;
> +	return ret;
> +}
> +
> +static __init bool regions_overlap(u32 s1, u32 e1, u32 s2, u32 e2)
> +{
> +	return e1 >= s2 && e2 >= s1;
> +}
> +
> +static __init bool overlaps_reserved_region(const void *fdt, u32 start,
> +				       u32 end, struct regions *regions)
> +{
> +	int subnode, len, i;
> +	u64 base, size;
> +
> +	/* check for overlap with /memreserve/ entries */
> +	for (i = 0; i < fdt_num_mem_rsv(fdt); i++) {
> +		if (fdt_get_mem_rsv(fdt, i, &base, &size) < 0)
> +			continue;
> +		if (regions_overlap(start, end, base, base + size))
> +			return true;
> +	}
> +
> +	if (regions->reserved_mem < 0)
> +		return false;
> +
> +	/* check for overlap with static reservations in /reserved-memory */
> +	for (subnode = fdt_first_subnode(fdt, regions->reserved_mem);
> +	     subnode >= 0;
> +	     subnode = fdt_next_subnode(fdt, subnode)) {
> +		const fdt32_t *reg;
> +		u64 rsv_end;
> +
> +		len = 0;
> +		reg = fdt_getprop(fdt, subnode, "reg", &len);
> +		while (len >= (regions->reserved_mem_addr_cells +
> +			       regions->reserved_mem_size_cells)) {
> +			base = fdt32_to_cpu(reg[0]);
> +			if (regions->reserved_mem_addr_cells == 2)
> +				base = (base << 32) | fdt32_to_cpu(reg[1]);
> +
> +			reg += regions->reserved_mem_addr_cells;
> +			len -= 4 * regions->reserved_mem_addr_cells;
> +
> +			size = fdt32_to_cpu(reg[0]);
> +			if (regions->reserved_mem_size_cells == 2)
> +				size = (size << 32) | fdt32_to_cpu(reg[1]);
> +
> +			reg += regions->reserved_mem_size_cells;
> +			len -= 4 * regions->reserved_mem_size_cells;
> +
> +			if (base >= regions->pa_end)
> +				continue;
> +
> +			rsv_end = min(base + size, (u64)U32_MAX);
> +
> +			if (regions_overlap(start, end, base, rsv_end))
> +				return true;
> +		}
> +	}
> +	return false;
> +}
> +
> +static __init bool overlaps_region(const void *fdt, u32 start,
> +				       u32 end, struct regions *regions)
> +{
> +	if (regions_overlap(start, end, regions->dtb_start,
> +			      regions->dtb_end))
> +		return true;
> +
> +	if (regions_overlap(start, end, regions->initrd_start,
> +			      regions->initrd_end))
> +		return true;
> +
> +	if (regions_overlap(start, end, regions->crash_start,
> +			      regions->crash_end))
> +		return true;
> +
> +	return overlaps_reserved_region(fdt, start, end, regions);
> +}
> +
> +static void __init get_crash_kernel(void *fdt, unsigned long size,
> +				struct regions *regions)
> +{
> +#ifdef CONFIG_KEXEC_CORE
> +	unsigned long long crash_size, crash_base;
> +	int ret;
> +
> +	ret = parse_crashkernel(early_command_line, size, &crash_size,
> +			&crash_base);
> +	if (ret != 0 || crash_size == 0)
> +		return;
> +	if (crash_base == 0)
> +		crash_base = KDUMP_KERNELBASE;
> +
> +	regions->crash_start = (unsigned long)crash_base;
> +	regions->crash_end = (unsigned long)(crash_base + crash_size);
> +
> +	DBG("crash_base=0x%llx crash_size=0x%llx\n", crash_base, crash_size);
> +#endif
> +}
> +
> +static void __init get_initrd_range(void *fdt, struct regions *regions)
> +{
> +	u64 start, end;
> +	int node, len;
> +	const __be32 *prop;
> +
> +	node = fdt_path_offset(fdt, "/chosen");
> +	if (node < 0)
> +		return;
> +
> +	prop = fdt_getprop(fdt, node, "linux,initrd-start", &len);
> +	if (!prop)
> +		return;
> +	start = of_read_number(prop, len / 4);
> +
> +	prop = fdt_getprop(fdt, node, "linux,initrd-end", &len);
> +	if (!prop)
> +		return;
> +	end = of_read_number(prop, len / 4);
> +
> +	regions->initrd_start = (unsigned long)start;
> +	regions->initrd_end = (unsigned long)end;
> +
> +	DBG("initrd_start=0x%llx  initrd_end=0x%llx\n", start, end);
> +}
> +
> +static __init unsigned long get_usable_offset(const void *fdt, struct regions *regions,
> +				unsigned long start)
> +{
> +	unsigned long pa;
> +	unsigned long pa_end;
> +
> +	for (pa = start; pa > regions->pa_start; pa -= SZ_16K) {
> +		pa_end = pa + regions->kernel_size;
> +		if (overlaps_region(fdt, pa, pa_end, regions))
> +			continue;
> +
> +		return pa;
> +	}
> +	return 0;
> +}
> +
> +static __init void get_cell_sizes(const void *fdt, int node, int *addr_cells,
> +			   int *size_cells)
> +{
> +	const int *prop;
> +	int len;
> +
> +	/*
> +	 * Retrieve the #address-cells and #size-cells properties
> +	 * from the 'node', or use the default if not provided.
> +	 */
> +	*addr_cells = *size_cells = 1;
> +
> +	prop = fdt_getprop(fdt, node, "#address-cells", &len);
> +	if (len == 4)
> +		*addr_cells = fdt32_to_cpu(*prop);
> +	prop = fdt_getprop(fdt, node, "#size-cells", &len);
> +	if (len == 4)
> +		*size_cells = fdt32_to_cpu(*prop);
> +}
> +
>   static unsigned long __init kaslr_choose_location(void *dt_ptr, phys_addr_t size,
>   					unsigned long kernel_sz)
>   {
> -	/* return a fixed offset of 64M for now */
> -	return 0x4000000;
> +	unsigned long offset, random;
> +	unsigned long ram, linear_sz;
> +	unsigned long kaslr_offset;
> +	u64 seed;
> +	struct regions regions;
> +	unsigned long index;
> +
> +	random = get_boot_seed(dt_ptr);
> +
> +	seed = get_tb() << 32;
> +	seed ^= get_tb();
> +	random = rotate_xor(random, &seed, sizeof(seed));
> +
> +	/*
> +	 * Retrieve (and wipe) the seed from the FDT
> +	 */
> +	seed = get_kaslr_seed(dt_ptr);
> +	if (seed)
> +		random = rotate_xor(random, &seed, sizeof(seed));
> +
> +	ram = min((phys_addr_t)__max_low_memory, size);
> +	ram = map_mem_in_cams(ram, CONFIG_LOWMEM_CAM_NUM, true);
> +	linear_sz = min(ram, (unsigned long)SZ_512M);
> +
> +	/* If the linear size is smaller than 64M, do not randmize */
> +	if (linear_sz < SZ_64M)
> +		return 0;
> +
> +	memset(&regions, 0, sizeof(regions));
> +
> +	/* check for a reserved-memory node and record its cell sizes */
> +	regions.reserved_mem = fdt_path_offset(dt_ptr, "/reserved-memory");
> +	if (regions.reserved_mem >= 0)
> +		get_cell_sizes(dt_ptr, regions.reserved_mem,
> +			       &regions.reserved_mem_addr_cells,
> +			       &regions.reserved_mem_size_cells);
> +
> +	regions.pa_start = 0;
> +	regions.pa_end = linear_sz;
> +	regions.dtb_start = __pa(dt_ptr);
> +	regions.dtb_end = __pa(dt_ptr) + fdt_totalsize(dt_ptr);
> +	regions.kernel_size = kernel_sz;
> +
> +	get_initrd_range(dt_ptr, &regions);
> +	get_crash_kernel(dt_ptr, ram, &regions);
> +
> +	/*
> +	 * Decide which 64M we want to start
> +	 * Only use the low 8 bits of the random seed
> +	 */
> +	index = random & 0xFF;
> +	index %= linear_sz / SZ_64M;
> +
> +	/* Decide offset inside 64M */
> +	if (index == 0) {
> +		offset = random % (SZ_64M - round_up(kernel_sz, SZ_16K) * 2);
> +		offset += round_up(kernel_sz, SZ_16K);
> +		offset = round_up(offset, SZ_16K);
> +	} else {
> +		offset = random % (SZ_64M - kernel_sz);
> +		offset = round_down(offset, SZ_16K);
> +	}
> +
> +	while (index >= 0) {
> +		offset = offset + index * SZ_64M;
> +		kaslr_offset = get_usable_offset(dt_ptr, &regions, offset);
> +		if (kaslr_offset)
> +			break;
> +		index--;
> +	}
> +
> +	/* Did not find any usable region? Give up randomize */
> +	if (index < 0)
> +		kaslr_offset = 0;
> +
> +	return kaslr_offset;
>   }
>   
>   /*
> @@ -58,6 +387,8 @@ notrace void __init kaslr_early_init(void *dt_ptr, phys_addr_t size)
>   
>   	kernel_sz = (unsigned long)_end - KERNELBASE;
>   
> +	kaslr_get_cmdline(dt_ptr);
> +
>   	offset = kaslr_choose_location(dt_ptr, size, kernel_sz);
>   
>   	if (offset == 0)
> 

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