Re: [PATCH v9 3/7] ima: kexec: skip IMA segment validation after kexec soft reboot

[Baoquan He <bhe@redhat.com>]

On 03/04/25 at 11:03am, steven chen wrote:
> The kexec_calculate_store_digests() function calculates and stores the
> digest of the segment during the 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 the kexec execute stage when a soft reboot is initiated. 
> Therefore, the digests should be updated for the IMA segment in the 
> normal case.
> 
> The content of memory segments carried over to the new kernel during the
> kexec systemcall can be changed at kexec 'execute' stage, but the size
> and the location of the memory segments cannot be changed at kexec
> 'execute' stage.
> 
> However, during the kexec execute stage, if kexec_calculate_store_digests()
> API is call to update the digest, it does not reuse the same memory segment
         ~ called
> allocated during the kexec 'load' stage and the new memory segment required
> cannot be transferred/mapped to the new kernel.
> 
> As a result, digest verification will fail in verify_sha256_digest() 
> after a kexec soft reboot into the new kernel. Therefore, the digest
> calculation/verification of the IMA segment needs to be skipped.
> 
> To address this, skip the calculation and storage of the digest for 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.
> 
> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Baoquan He <bhe@redhat.com> 
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Dave Young <dyoung@redhat.com>
> Signed-off-by: steven chen <chenste@linux.microsoft.com>
> Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
> 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(+)

Other than the nit, LGTM.

> 
> diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> index 7d6b12f8b8d0..107e726f2ef3 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 6195df128482..0465beca8867 100644
> --- a/security/integrity/ima/ima_kexec.c
> +++ b/security/integrity/ima/ima_kexec.c
> @@ -169,6 +169,7 @@ 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");
> @@ -179,6 +180,8 @@ 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
> -- 
> 2.25.1
>