Re: [PATCH v5 1/7] kexec_file: allow to place kexec_buf randomly

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Jun 07, 2024 at 08:26:11PM +0800, Coiby Xu wrote:
> Currently, kexec_buf is placed in order which means for the same
> machine, the info in the kexec_buf is always located at the same
> position each time the machine is booted. This may cause a risk for
> sensitive information like LUKS volume key. Now struct kexec_buf has a
> new field random which indicates it's supposed to be placed in a random
> position.
> 
> Suggested-by: Jan Pazdziora <jpazdziora@redhat.com>
> Signed-off-by: Coiby Xu <coxu@redhat.com>
> ---
>  include/linux/kexec.h |  4 ++++
>  kernel/kexec_file.c   | 21 +++++++++++++++++++++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index f0e9f8eda7a3..c45bfc727737 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -171,6 +171,7 @@ int kexec_image_post_load_cleanup_default(struct kimage *image);
>   * @buf_min:	The buffer can't be placed below this address.
>   * @buf_max:	The buffer can't be placed above this address.
>   * @top_down:	Allocate from top of memory.
> + * @random:	Place the buffer at a random position.
>   */
>  struct kexec_buf {
>  	struct kimage *image;
> @@ -182,6 +183,9 @@ struct kexec_buf {
>  	unsigned long buf_min;
>  	unsigned long buf_max;
>  	bool top_down;
> +#ifdef CONFIG_CRASH_DUMP
> +	bool random;
> +#endif

Why is the ifdef needed?

>  };
>  
>  int kexec_load_purgatory(struct kimage *image, struct kexec_buf *kbuf);
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 3d64290d24c9..f7538d8f67e0 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -25,6 +25,9 @@
>  #include <linux/elfcore.h>
>  #include <linux/kernel.h>
>  #include <linux/kernel_read_file.h>
> +#ifdef CONFIG_CRASH_DUMP
> +#include <linux/prandom.h>
> +#endif

No ifdef in .c files please.  This should not be an issue.

>  #include <linux/syscalls.h>
>  #include <linux/vmalloc.h>
>  #include "kexec_internal.h"
> @@ -437,6 +440,18 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
>  	return ret;
>  }
>  
> +#ifdef CONFIG_CRASH_DUMP
> +static unsigned long kexec_random_start(unsigned long start, unsigned long end)
> +{
> +	unsigned long temp_start;
> +	unsigned short i;
> +
> +	get_random_bytes(&i, sizeof(unsigned short));
> +	temp_start = start + (end - start) / USHRT_MAX * i;
> +	return temp_start;
> +}
> +#endif

This #ifdef should be handled properly in the .h file.

> +
>  static int locate_mem_hole_top_down(unsigned long start, unsigned long end,
>  				    struct kexec_buf *kbuf)
>  {
> @@ -445,6 +460,10 @@ static int locate_mem_hole_top_down(unsigned long start, unsigned long end,
>  
>  	temp_end = min(end, kbuf->buf_max);
>  	temp_start = temp_end - kbuf->memsz + 1;
> +#ifdef CONFIG_CRASH_DUMP
> +	if (kbuf->random)
> +		temp_start = kexec_random_start(temp_start, temp_end);
> +#endif

Same with this.

And why do you need the boolean at all, why not just have
kexec_random_start() handle this properly for you?

thanks,

greg k-h