Subject: Re: [PATCH v16 16/16] arm64: kexec_file: add kaslr support

From: Will Deacon <[email-masked]>
Date: Fri, 30 Nov 2018 13:19:44 +0000

On Thu, Nov 15, 2018 at 02:52:55PM +0900, AKASHI Takahiro wrote:
> Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual
> address randomization, at secondary kernel boot. We always do this as
> it will have no harm on kaslr-incapable kernel.
> 
> We don't have any "switch" to turn off this feature directly, but still
> can suppress it by passing "nokaslr" as a kernel boot argument.
> 
> Signed-off-by: AKASHI Takahiro <[email-masked]>
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> ---
>  arch/arm64/kernel/machine_kexec_file.c | 46 +++++++++++++++++++++++++-
>  1 file changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> index ab296b98d633..a0a730bd9be6 100644
> --- a/arch/arm64/kernel/machine_kexec_file.c
> +++ b/arch/arm64/kernel/machine_kexec_file.c
> @@ -16,6 +16,7 @@
>  #include <linux/libfdt.h>
>  #include <linux/memblock.h>
>  #include <linux/of_fdt.h>
> +#include <linux/random.h>
>  #include <linux/slab.h>
>  #include <linux/string.h>
>  #include <linux/types.h>
> @@ -28,6 +29,7 @@
>  #define FDT_PSTR_INITRD_STA	"linux,initrd-start"
>  #define FDT_PSTR_INITRD_END	"linux,initrd-end"
>  #define FDT_PSTR_BOOTARGS	"bootargs"
> +#define FDT_PSTR_KASLR_SEED	"kaslr-seed"
>  
>  const struct kexec_file_ops * const kexec_file_loaders[] = {
>  	&kexec_image_ops,
> @@ -46,11 +48,38 @@ int arch_kimage_file_post_load_cleanup(struct kimage *image)
>  	return kexec_image_post_load_cleanup_default(image);
>  }
>  
> +/* crng needs to have been initialized for providing kaslr-seed */
> +static int random_ready;
> +
> +static void random_ready_notified(struct random_ready_callback *unused)
> +{
> +	random_ready = 1;
> +}
> +
> +static struct random_ready_callback random_ready_cb = {
> +	.func = random_ready_notified,
> +};
> +
> +static __init int init_random_ready_cb(void)
> +{
> +	int ret;
> +
> +	ret = add_random_ready_callback(&random_ready_cb);
> +	if (ret == -EALREADY)
> +		random_ready = 1;
> +	else if (ret)
> +		pr_warn("failed to add a callback for random_ready\n");
> +
> +	return 0;
> +}
> +late_initcall(init_random_ready_cb)

Why can't we just call crng_ready()?

> +
>  static int setup_dtb(struct kimage *image,
>  		     unsigned long initrd_load_addr, unsigned long initrd_len,
>  		     char *cmdline, void *dtb)
>  {
>  	int nodeoffset;
> +	u64 value;
>  	int ret;
>  
>  	nodeoffset = fdt_path_offset(dtb, "/chosen");
> @@ -106,12 +135,27 @@ static int setup_dtb(struct kimage *image,
>  			return -EINVAL;
>  	}
>  
> +	/* add kaslr-seed */
> +	ret = fdt_delprop(dtb, nodeoffset, FDT_PSTR_KASLR_SEED);
> +	if (ret && (ret != -FDT_ERR_NOTFOUND))
> +		return -EINVAL;
> +
> +	if (random_ready) {
> +		get_random_bytes(&value, sizeof(value));

get_random_u64() ?

> +		ret = fdt_setprop_u64(dtb, nodeoffset, FDT_PSTR_KASLR_SEED,
> +							value);
> +		if (ret)
> +			return (ret == -FDT_ERR_NOSPACE ? -ENOMEM : -EINVAL);
> +	} else {

Wouldn't we be better off preserving the previous seed here, if it was
present?

> +		pr_notice("kaslr-seed won't be fed\n");

"fed" is probably not the right word here.

Will

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