[PATCH v9 1/8] kexec_file: allow to place kexec_buf randomly

[Breno Leitao]

Hello Coiby,

On Fri, May 02, 2025 at 09:12:35AM +0800, Coiby Xu wrote:
> +static inline void kexec_random_range_start(unsigned long start,
> +					    unsigned long end,
> +					    struct kexec_buf *kbuf,
> +					    unsigned long *temp_start)
> +{
> +	unsigned short i;
> +
> +	if (kbuf->random) {
> +		get_random_bytes(&i, sizeof(unsigned short));
> +		*temp_start = start + (end - start) / USHRT_MAX * i;
> +	}
> +}

On arm64, I am getting the following UBSAN warning when accessing
kbuf->random:

[   32.362428] ------------[ cut here ]------------
[   32.362488] UBSAN: invalid-load in ./include/linux/kexec.h:210:10
[   32.362649] load of value 252 is not a valid value for type '_Bool'

and line 210 is your `if (kbuf->random)`.

Basically kbuf was not initialized in arm hosts, and probably has
garbage.

I am wondering if we should have something like , while the support for arm64 is
not done:

commit 2608bd8c26b62a9a7cc50106e93d3a1ffb1e1188
Author: Breno Leitao <leitao at debian.org>
Date:   Thu Aug 21 04:11:21 2025 -0700

    Initialize the random field of kbuf to zero in the ARM64 kexec image loader

    Ads an explicit initialization for the random member of the kbuf
    structure within the image_load function in
    arch/arm64/kernel/kexec_image.c. Setting kbuf.random to zero ensures
    a deterministic and clean starting state for the buffer used during
    kernel image loading, avoiding this UBSAN issue later, when kbuf.random
    is read.

            [   32.362488] UBSAN: invalid-load in ./include/linux/kexec.h:210:10
            [   32.362649] load of value 252 is not a valid value for type '_Bool'

    Signed-off-by: Breno Leitao <leitao at debian.org>

diff --git a/arch/arm64/kernel/kexec_image.c b/arch/arm64/kernel/kexec_image.c
index 532d72ea42ee8..287b25e674d76 100644
--- a/arch/arm64/kernel/kexec_image.c
+++ b/arch/arm64/kernel/kexec_image.c
@@ -76,6 +76,7 @@ static void *image_load(struct kimage *image,
 	kbuf.buf_min = 0;
 	kbuf.buf_max = ULONG_MAX;
 	kbuf.top_down = false;
+	kbuf.random = 0;

 	kbuf.buffer = kernel;
 	kbuf.bufsz = kernel_len;