Re: [PATCH v4 6/7] ima: make the kexec extra memory configurable

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

On 1/22/24 13:38, Tushar Sugandhi wrote:
> The extra memory allocated for carrying the IMA measurement list across
> kexec is hardcoded as half a PAGE.  Make it configurable.
> 
> Define a Kconfig option, IMA_KEXEC_EXTRA_MEMORY_KB, to configure the
> extra memory (in kb) to be allocated for IMA measurements added during
> kexec soft reboot.  Ensure the default value of the option is set such
> that extra half a page of memory for additional measurements is allocated
> to maintain backwards compatibility.
> 
> Update ima_add_kexec_buffer() function to allocate memory based on the
> Kconfig option value, rather than the currently hardcoded one.
> 
> Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
> ---
>   security/integrity/ima/Kconfig     | 11 +++++++++++
>   security/integrity/ima/ima_kexec.c | 15 ++++++++++-----
>   2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
> index 60a511c6b583..fc103288852b 100644
> --- a/security/integrity/ima/Kconfig
> +++ b/security/integrity/ima/Kconfig
> @@ -338,3 +338,14 @@ config IMA_DISABLE_HTABLE
>   	default n
>   	help
>   	   This option disables htable to allow measurement of duplicate records.
> +
> +config IMA_KEXEC_EXTRA_MEMORY_KB
> +	int
> +	depends on IMA && IMA_KEXEC
> +	default 0
> +	help
> +	  IMA_KEXEC_EXTRA_MEMORY_KB determines the extra memory to be
> +	  allocated (in kb) for IMA measurements added during kexec soft reboot.
> +	  If set to the default value, an extra half page of memory for
> +	  additional measurements will be allocated to maintain backwards
> +	  compatibility.

Is there really an issue with 'backwards compatibility' that the user 
needs to know about ? From looking at the code it seems more important 
that a bit of additional memory is reserved now to fit the kexec 'load' 
and 'exec' critical data events but that's not 'backwards compatibility'.

Also mention this as a guidance on either how kexec load+exec need to be 
used or as a hint that it may potentially require a lot of memory? :

The amount of extra memory that is necessary to carry all measurements 
across a kexec depends on memory requirements of the measurements taken 
between the kexec 'load' and 'exec' stages. The more measurements are 
taken, the more extra memory is required. Large amounts of extra memory 
can be avoided by running kexec 'load' and 'exec' in short sequence.

> diff --git a/security/integrity/ima/ima_kexec.c b/security/integrity/ima/ima_kexec.c
> index f26b5b342d84..c126aa6494e9 100644
> --- a/security/integrity/ima/ima_kexec.c
> +++ b/security/integrity/ima/ima_kexec.c
> @@ -121,6 +121,7 @@ void ima_add_kexec_buffer(struct kimage *image)
>   				  .buf_min = 0, .buf_max = ULONG_MAX,
>   				  .top_down = true };
>   	unsigned long binary_runtime_size;
> +	unsigned long extra_size;
>   
>   	/* use more understandable variable names than defined in kbuf */
>   	void *kexec_buffer = NULL;
> @@ -128,15 +129,19 @@ void ima_add_kexec_buffer(struct kimage *image)
>   	int ret;
>   
>   	/*
> -	 * Reserve an extra half page of memory for additional measurements
> -	 * added during the kexec load.
> +	 * Reserve extra memory for measurements added during kexec.
>   	 */
> -	binary_runtime_size = ima_get_binary_runtime_size();
> +	if (CONFIG_IMA_KEXEC_EXTRA_MEMORY_KB <= 0)
> +		extra_size = PAGE_SIZE / 2;
> +	else
> +		extra_size = CONFIG_IMA_KEXEC_EXTRA_MEMORY_KB * 1024;
> +	binary_runtime_size = ima_get_binary_runtime_size() + 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");

Code LGTM