Re: [PATCH v8 3/7] ima: kexec: skip IMA segment validation after kexec soft reboot

[Mimi Zohar <zohar@linux.ibm.com>]

On Tue, 2025-02-18 at 14:54 -0800, steven chen wrote:
> kexec_calculate_store_digests() calculates and stores the digest of the
> segment at kexec_file_load syscall where the IMA segment is also
> allocated.  With this series, the IMA segment will be updated with the
> measurement log at kexec execute stage when soft reboot is initiated. 
> Therefore, it may fail digest verification in verify_sha256_digest() 
> after kexec soft reboot into the new kernel. Therefore, the digest 
> calculation/verification of the IMA segment needs to be skipped.
> 
> Skip the calculating and storing digest of the IMA segment in
> kexec_calculate_store_digests() so that it is not added to the
> 'purgatory_sha_regions'.
> 
> Since verify_sha256_digest() only verifies 'purgatory_sha_regions',
> no change is needed in verify_sha256_digest() in this context.
> 
> With this change, the IMA segment is not included in the digest
> calculation, storage, and verification.

Basically you're saying because the hash verification will fail, don't include
the IMA buffer.  What's missing is the reason for not caring whether the IMA
hash is included or not.

I understand this is the best we can do without making some major kexec changes.

> 
> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
> Signed-off-by: steven chen <chenste@linux.microsoft.com>
> Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>

After updating the patch description,

Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>

> ---
>  include/linux/kexec.h              |  3 +++
>  kernel/kexec_file.c                | 22 ++++++++++++++++++++++
>  security/integrity/ima/ima_kexec.c |  3 +++
>  3 files changed, 28 insertions(+)
> 
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index 4dbf806bccef..bd554ced9fb2 100644
> --- a/include/linux/kexec.h
> +++ b/include/linux/kexec.h
> @@ -362,6 +362,9 @@ struct kimage {
>  
>  	phys_addr_t ima_buffer_addr;
>  	size_t ima_buffer_size;
> +
> +	unsigned long ima_segment_index;
> +	bool is_ima_segment_index_set;
>  #endif
>  
>  	/* Core ELF header buffer */
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 3eedb8c226ad..606132253c79 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -38,6 +38,21 @@ void set_kexec_sig_enforced(void)
>  }
>  #endif
>  
> +#ifdef CONFIG_IMA_KEXEC
> +static bool check_ima_segment_index(struct kimage *image, int i)
> +{
> +	if (image->is_ima_segment_index_set && i == image->ima_segment_index)
> +		return true;
> +	else
> +		return false;
> +}
> +#else
> +static bool check_ima_segment_index(struct kimage *image, int i)
> +{
> +	return false;
> +}
> +#endif
> +
>  static int kexec_calculate_store_digests(struct kimage *image);
>  
>  /* Maximum size in bytes for kernel/initrd files. */
> @@ -764,6 +779,13 @@ static int kexec_calculate_store_digests(struct kimage
> *image)
>  		if (ksegment->kbuf == pi->purgatory_buf)
>  			continue;
>  
> +		/*
> +		 * Skip the segment if ima_segment_index is set and matches
> +		 * the current index
> +		 */
> +		if (check_ima_segment_index(image, i))
> +			continue;
> +
>  		ret = crypto_shash_update(desc, ksegment->kbuf,
>  					  ksegment->bufsz);
>  		if (ret)
> diff --git a/security/integrity/ima/ima_kexec.c
> b/security/integrity/ima/ima_kexec.c
> index 89088f1fa989..704676fa6615 100644
> --- a/security/integrity/ima/ima_kexec.c
> +++ b/security/integrity/ima/ima_kexec.c
> @@ -160,6 +160,7 @@ void ima_add_kexec_buffer(struct kimage *image)
>  	kbuf.buffer = kexec_buffer;
>  	kbuf.bufsz = kexec_buffer_size;
>  	kbuf.memsz = kexec_segment_size;
> +	image->is_ima_segment_index_set = false;
>  	ret = kexec_add_buffer(&kbuf);
>  	if (ret) {
>  		pr_err("Error passing over kexec measurement buffer.\n");
> @@ -170,6 +171,8 @@ void ima_add_kexec_buffer(struct kimage *image)
>  	image->ima_buffer_addr = kbuf.mem;
>  	image->ima_buffer_size = kexec_segment_size;
>  	image->ima_buffer = kexec_buffer;
> +	image->ima_segment_index = image->nr_segments - 1;
> +	image->is_ima_segment_index_set = true;
>  
>  	/*
>  	 * kexec owns kexec_buffer after kexec_add_buffer() is called