[PATCH v2] spl: Add callback for preprocessing loaded FIT header before parsing

[Alex G.]

On 3/9/21 5:55 PM, Farhan Ali wrote:
> This change adds a callback for preprocessing the FIT header before
> it is parsed. There are 3 main reasons for this callback:
> 
> (1) If a vulnerability is discovered in the FIT parsing/loading code,
> or libfdt, this callback allows users to scan the FIT header for
> specific exploit signatures and prevent flashing/booting of the image
> 
> (2) If users want to implement a single signature which covers the
> entire FIT header, which is then appended to the end of the header,
> then this callback can be used to authenticate that signature.
> 
> (3) If users want to check the FIT header contents against specific
> metadata stored outside the FIT header, then this callback allows
> them to do that.

Hi Fahran,

This patch describes "how" you're trying to achieve it, but "what" you 
want to achieve. I'll get later into why I think the "how" is 
fundamentally flawed.

There should be at least a use case implemented in this series. When 
someone notices an empty function that isn't used, the first instinct 
would be to submit a patch to remove it. But more importantly, seeing an 
actual example of "what" you are trying to achieve will help others 
suggest a better way on "how" to achieve it.

Second issue is that spl_simple_fit_read() is intended to bring a FIT 
image to memory. If you need to make decisions on the content of that 
image, then spl_simple_fit_read() is the wrong place to do it. A better 
place might be spl_simple_fit_parse().

The third issue is that whatever the end goal is, you're trying to 
achieve it with weak functions. Weak functions aren't always bad. There 
are a limited number of cases where they work very well for the purpose 
-- I've even used them before. But to introduce a weak function, a 
really strong argument is needed. Maybe you have it, but that argument 
needs to be made clear.

Let's assume board 'c' implements this. Then later someone with board 
'd' implements this at the SOC level. Does board 'c' get the old 
implementation, or the new? Things become ambiguous in everything but 
the simplest of cases.

A more elegant way would be to have a proper interface to hook into the 
FIT processing. That could be done by a function pointer, ops structure, 
or perhaps new UCLASS (Simon?).

Alex




> Signed-off-by: Farhan Ali <farhan.ali at broadcom.com>
> Cc: Simon Glass <sjg at chromium.org>
> Cc: Alexandru Gagniuc <mr.nuke.me at gmail.com>
> Cc: Marek Vasut <marex at denx.de>
> Cc: Michal Simek <michal.simek at xilinx.com>
> Cc: Philippe Reynes <philippe.reynes at softathome.com>
> Cc: Samuel Holland <samuel at sholland.org>
> 
> ---
> Changes for v2:
>     - Callback now returns a value
>     - Added a log message on failure
> ---
>   common/spl/spl_fit.c | 22 +++++++++++++++++++++-
>   1 file changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/common/spl/spl_fit.c b/common/spl/spl_fit.c
> index 75c8ff0..01aee1c 100644
> --- a/common/spl/spl_fit.c
> +++ b/common/spl/spl_fit.c
> @@ -43,6 +43,14 @@ __weak ulong board_spl_fit_size_align(ulong size)
>   	return size;
>   }
>   
> +__weak int board_spl_fit_pre_load(struct spl_load_info *load_info,
> +				  const void *fit,
> +				  ulong start_sector,
> +				  ulong loaded_sector_count)
> +{
> +	return 0;
> +}
> +
>   static int find_node_from_desc(const void *fit, int node, const char *str)
>   {
>   	int child;
> @@ -527,6 +535,7 @@ static int spl_simple_fit_read(struct spl_fit_info *ctx,
>   	unsigned long count, size;
>   	int sectors;
>   	void *buf;
> +	int rc = 0;
>   
>   	/*
>   	 * For FIT with external data, figure out where the external images
> @@ -552,7 +561,18 @@ static int spl_simple_fit_read(struct spl_fit_info *ctx,
>   	debug("fit read sector %lx, sectors=%d, dst=%p, count=%lu, size=0x%lx\n",
>   	      sector, sectors, buf, count, size);
>   
> -	return (count == 0) ? -EIO : 0;
> +	if (count) {
> +		/* preprocess loaded fit header before parsing and loading binaries */
> +		rc = board_spl_fit_pre_load(info, fit_header, sector, sectors);
> +		if (rc) {
> +			debug("%s: fit header pre processing failed. rc=%d\n",
> +			      __func__, rc);
> +		}
> +	} else {
> +		rc = -EIO;
> +	}
> +
> +	return rc;
>   }
>   
>   static int spl_simple_fit_parse(struct spl_fit_info *ctx)
>