Re: [PATCH 03/10] ima: allocate buffer at kexec load to hold ima measurements

[Stefan Berger <stefanb@linux.ibm.com>]

On 7/3/23 17:57, Tushar Sugandhi wrote:
> The IMA subsystem needs a dedicated mechanism to reserve extra memory for
> measurements added between the kexec 'load' and kexec 'execute'.
> 
> Update ima_add_kexec_buffer to allocate a buffer of a sufficient size
> taking ima binary runtime measurements size, size of ima_kexec_hdr, and
> IMA_KEXEC_EXTRA_SIZE into account.  Adjust the kexec_segment_size to align
> to the PAGE_SIZE.  Call ima_allocate_buf_at_kexec_load() to allocate the
> buffer.
> 
> This patch assumes the extra space defined (IMA_KEXEC_EXTRA_SIZE) is
> sufficient to handle the additional measurements.  This should be as per
> the system requirements and based on the number of additional measurements
> expected during the window from kexec 'load' to kexec 'execute'.

This could be the most problematic part if the 'execute' happens much later
than the 'load' with lots of measurement activity in between. I am wondering
whether not doing anything at 'load' time and doing the whole work at 'execute' time
wouldn't be the right thing to do ?

Otherwise, if we wanted the work to be split as you suggest, could you
- krealloc the src_pages (now in kimage_map_segment) to add space for a few more pages needed for the additional measurements
- add those few more pages to src_pages
- vunmap the previous mapping
- vmap the extended src_pages array
- update ima_kexec_file.buf with the diff between the new and old vmap'ed addresses
- append to the existing log

This presumably would help resolve this potential issue.

The src_pages is currently not kfree'd -- may be a memory leak.

Regards,
    Stefan

> 
> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
> ---
>   security/integrity/ima/ima.h       |  2 ++
>   security/integrity/ima/ima_kexec.c | 21 ++++++++++-----------
>   2 files changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index c29db699c996..2ffda9449b9b 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -43,6 +43,8 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8, TPM_PCR10 = 10 };
>   
>   #define NR_BANKS(chip) ((chip != NULL) ? chip->nr_allocated_banks : 0)
>   
> +#define IMA_KEXEC_EXTRA_SIZE (16 * PAGE_SIZE)
> +
>   /* current content of the policy */
>   extern int ima_policy_flag;
>   
> diff --git a/security/integrity/ima/ima_kexec.c b/security/integrity/ima/ima_kexec.c
> index 858b67689701..7deb8df31485 100644
> --- a/security/integrity/ima/ima_kexec.c
> +++ b/security/integrity/ima/ima_kexec.c
> @@ -188,31 +188,30 @@ void ima_add_kexec_buffer(struct kimage *image)
>   	/* use more understandable variable names than defined in kbuf */
>   	void *kexec_buffer = NULL;
>   	size_t kexec_buffer_size;
> -	size_t kexec_segment_size;
>   	int ret;
>   
>   	/*
> -	 * Reserve an extra half page of memory for additional measurements
> -	 * added during the kexec load.
> +	 * Reserve extra memory for measurements added in the window from
> +	 * kexec 'load' to kexec 'execute'.
>   	 */
> -	binary_runtime_size = ima_get_binary_runtime_size();
> +	binary_runtime_size = ima_get_binary_runtime_size() +
> +			      sizeof(struct ima_kexec_hdr) +
> +			      IMA_KEXEC_EXTRA_SIZE;
> +
>   	if (binary_runtime_size >= ULONG_MAX - PAGE_SIZE)
>   		kexec_segment_size = ULONG_MAX;
>   	else
> -		kexec_segment_size = ALIGN(ima_get_binary_runtime_size() +
> -					   PAGE_SIZE / 2, PAGE_SIZE);
> +		kexec_segment_size = ALIGN(binary_runtime_size, PAGE_SIZE);
> +
>   	if ((kexec_segment_size == ULONG_MAX) ||
>   	    ((kexec_segment_size >> PAGE_SHIFT) > totalram_pages() / 2)) {
>   		pr_err("Binary measurement list too large.\n");
>   		return;
>   	}
>   
> -	ima_dump_measurement_list(&kexec_buffer_size, &kexec_buffer,
> -				  kexec_segment_size);
> -	if (!kexec_buffer) {
> -		pr_err("Not enough memory for the kexec measurement buffer.\n");
> +	ret = ima_allocate_buf_at_kexec_load();
> +	if (ret < 0)
>   		return;
> -	}
>   
>   	kbuf.buffer = kexec_buffer;
>   	kbuf.bufsz = kexec_buffer_size;