Re: [PATCH v4 1/7] kexec_file: allow to place kexec_buf randomly

Baoquan He
2024-06-04T07:41:45Z

On 05/23/24 at 01:04pm, 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 | 2 ++
> kernel/kexec_file.c | 15 +++++++++++++++
> 2 files changed, 17 insertions(+)
>
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index f0e9f8eda7a3..cc81b8a903ab 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 +183,7 @@ struct kexec_buf {
> unsigned long buf_min;
> unsigned long buf_max;
> bool top_down;
> + bool random;
> };
>
> 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..06b77f9ac4cc 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -25,6 +25,7 @@
> #include <linux/elfcore.h>
> #include <linux/kernel.h>
> #include <linux/kernel_read_file.h>
> +#include <linux/prandom.h>
> #include <linux/syscalls.h>
> #include <linux/vmalloc.h>
> #include "kexec_internal.h"
> @@ -437,6 +438,16 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> return ret;
> }
>
> +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;
> +}
> +
> static int locate_mem_hole_top_down(unsigned long start, unsigned long end,
> struct kexec_buf *kbuf)
> {
> @@ -445,6 +456,8 @@ 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;
> + if (kbuf->random)
> + temp_start = kexec_random_start(temp_start, temp_end);

As we discussed before, this need be limited in kdump scope, seems v4
doesn't include the change.

>
> do {
> /* align down start */
> @@ -482,6 +495,8 @@ static int locate_mem_hole_bottom_up(unsigned long start, unsigned long end,
> unsigned long temp_start, temp_end;
>
> temp_start = max(start, kbuf->buf_min);
> + if (kbuf->random)
> + temp_start = kexec_random_start(temp_start, end);
>
> do {
> temp_start = ALIGN(temp_start, kbuf->buf_align);
> --
> 2.45.0
>