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

[Coiby Xu]

On Thu, Aug 21, 2025 at 04:15:53AM -0700, Breno Leitao wrote:
>Hello Coiby,

Hi Breno,

>
>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.

Thank for explaining the problem to me!

>
>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;
>

And also thanks for posing a fix! The patch LGTM. Can you add a Fixes
tag 'Fixes: bf454ec31add ("kexec_file: allow to place kexec_buf
randomly")' and then send it to kexec at lists.infradead.org? Thanks!

-- 
Best regards,
Coiby