On Sat, May 09, 2020 at 06:38:33PM +0200, Heinrich Schuchardt wrote:
On 5/9/20 6:13 PM, Patrick Wildt wrote:
On i.MX platforms board_spl_fit_post_load() can check the loaded SPL image for authenticity using its HAB engine. U-Boot's SPL mechanism allows booting images from other sources as well, but in the current setup the SPL would just hang if it encounters an image that does not pass scrutiny. Allowing the function to return an error, allows the SPL to try booting from another source as a fallback instead of ending up as a brick.
Signed-off-by: Patrick Wildt patrick@blueri.se
Could an intruder abuse this by destroying a signed image and providing an unsigned image on a source under his control?
Best regards
Heinrich
Sure, let's think about it here. Maybe you have some more thoughts to add.
First of all, the SPL goes through all the boot devices, and if there's none to find with an image, it will hang. It will hang like it does before the diff. The only difference is that it tries additional sources before hanging. Thus the attacker, unless he can exploit it in his first trial, or is able to force a reset, must have some access to reset the machine to have it boot and try again. This seems like he must have some kind of local or remote phyiscal access.
Let's assume the image is on the network or on another remote medium. Then I guess the attacker will just try to attack that medium, and the alternate boot sources won't make a difference.
I guess that means we should focus on local sources. I think we can also ignore a removable SD card, since he can just put in another one and try again.
So maybe let's think about SPI flash and eMMC, soldered on, not directly accessible. If he has physical access, I guess he could open up the box and desolder a few pins, or add some voltage here and there to try and disrupt the bootup. But, then maybe it's easier to just desolder the whole SPI/eMMC and add his own.
But what if he doesn't have that access? If he's remote? Ok, he will probably have to exploit the daemon (webserver or whatever) to gain some code execution. Then he'll try and become root, so he can access the disks. Then I figure he'll try and overwrite or remove the image. With this, on the next reboot it will (hopefully) fail to boot, unless he already has an exploit, then my patch won't make a difference.
I figure the real issue could be that when the attacker has physical access, manages to remove/replace the image with a fallback to load from a device like an SD card, that it's now easier for him to try and find an exploit.
Am I missing something?
Best regards, Patrick