linux-kernel - Re: [PATCH v2 07/10] powerpc/fsl_booke/32: randomize the kernel image offset

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Message-ID: <b41c4650-ef30-6f02-d5b3-bc95c5ad3ce0@c-s.fr>
Date:   Tue, 30 Jul 2019 11:44:42 +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,
        zhaohongjiang@...wei.com
Subject: Re: [PATCH v2 07/10] powerpc/fsl_booke/32: randomize the kernel image
 offset



Le 30/07/2019 à 09:42, 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 | 334 +++++++++++++++++++++++++++++-
>   1 file changed, 332 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kaslr_booke.c b/arch/powerpc/kernel/kaslr_booke.c
> index 960bce4aa8b9..0bb02e45b928 100644
> --- a/arch/powerpc/kernel/kaslr_booke.c
> +++ b/arch/powerpc/kernel/kaslr_booke.c
> @@ -23,6 +23,8 @@
>   #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>
> @@ -34,15 +36,341 @@
>   #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 __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(boot_command_line, cmdline, COMMAND_LINE_SIZE);

boot_command_line is set by early_init_devtree() in 
arch/powerpc/kernel/prom.c
Is that too late for you ?

If so, what about calling early_init_dt_scan_chosen() instead of recoding ?

Christophe