SPL -> U-Boot Chain of Trust

I'm interested in getting secure device support going, but it seems like we should need more than that, ie something to keep the chain of trust going.

Quoted 3 lines · Tom Rini

I'm interested in getting secure device support going, but it seems like we should need more than that, ie something to keep the chain of trust going.

Tom et al.,
I just saw your reply to Vitaly's email and I'm actually just looking into something along the lines you brought up but I didn't want to hijack that discussion so here's a new thread.

As for the chain of trust for ARMv7, my understanding is that when using a combination of SPL and U-Boot there will always be a vendor- specific initial boot (ROM) code that authenticates SPL, and then there will need to be some code inserted into SPL that authenticates U-Boot after it's loaded (for example by using some secure ROM API call and such).

So I was looking into if there is already some generic framework for this in U-Boot but didn't see anything obvious. One "easy" way would be to add a simple call to an authentication routine to board_init_r (u-boot/common/spl/spl.c) but let's say we add such a call for TI or other vendor's stuff I suppose this would not scale very well.

But what about adding one generic call to a default authentication function declared as __weak for spl_image that doesn't do anything, but can be overwritten in vendor-specific files to provide means of authenticating spl_image. Would this be a good approach?

Beyond that I was reviewing some of the awesome work from the Chromium team and I think on ARMv7 after we get MLO to authenticate U-Boot everything beyond that is already looking very solid and thorough (with FIT, DTB/Kernel and initramfs authentication).

Thanks and Regards,

--
Andreas Dannenberg
Texas Instruments Inc

Hi Andreas,

Quoted 30 lines · Andreas Dannenberg

Quoted 3 lines · Tom Rini

I'm interested in getting secure device support going, but it seems like we should need more than that, ie something to keep the chain of trust going.

Tom et al.,
I just saw your reply to Vitaly's email and I'm actually just looking into something along the lines you brought up but I didn't want to hijack that discussion so here's a new thread.

As for the chain of trust for ARMv7, my understanding is that when using a combination of SPL and U-Boot there will always be a vendor- specific initial boot (ROM) code that authenticates SPL, and then there will need to be some code inserted into SPL that authenticates U-Boot after it's loaded (for example by using some secure ROM API call and such).

So I was looking into if there is already some generic framework for this in U-Boot but didn't see anything obvious. One "easy" way would be to add a simple call to an authentication routine to board_init_r (u-boot/common/spl/spl.c) but let's say we add such a call for TI or other vendor's stuff I suppose this would not scale very well.

But what about adding one generic call to a default authentication function declared as __weak for spl_image that doesn't do anything, but can be overwritten in vendor-specific files to provide means of authenticating spl_image. Would this be a good approach?

Quoted 4 lines

Beyond that I was reviewing some of the awesome work from the Chromium team and I think on ARMv7 after we get MLO to authenticate U-Boot everything beyond that is already looking very solid and thorough (with FIT, DTB/Kernel and initramfs authentication).

It should be possible to use this from SPL, if you can enable FIT in SPL. The current implementation does not support verification, and is deliberately cut down. See common/spl/spl_fit.c.

but you could perhaps
provide an option to use the full U-Boot implementation instead.

Hi Simon, thanks for the feedback. Additional comments inlined...

Quoted 33 lines · Simon Glass

Hi Andreas,

Quoted 30 lines · Andreas Dannenberg

Quoted 3 lines · Tom Rini

I'm interested in getting secure device support going, but it seems like we should need more than that, ie something to keep the chain of trust going.

Tom et al.,
I just saw your reply to Vitaly's email and I'm actually just looking into something along the lines you brought up but I didn't want to hijack that discussion so here's a new thread.

As for the chain of trust for ARMv7, my understanding is that when using a combination of SPL and U-Boot there will always be a vendor- specific initial boot (ROM) code that authenticates SPL, and then there will need to be some code inserted into SPL that authenticates U-Boot after it's loaded (for example by using some secure ROM API call and such).

So I was looking into if there is already some generic framework for this in U-Boot but didn't see anything obvious. One "easy" way would be to add a simple call to an authentication routine to board_init_r (u-boot/common/spl/spl.c) but let's say we add such a call for TI or other vendor's stuff I suppose this would not scale very well.

But what about adding one generic call to a default authentication function declared as __weak for spl_image that doesn't do anything, but can be overwritten in vendor-specific files to provide means of authenticating spl_image. Would this be a good approach?

Quoted 8 lines

Quoted 4 lines

Beyond that I was reviewing some of the awesome work from the Chromium team and I think on ARMv7 after we get MLO to authenticate U-Boot everything beyond that is already looking very solid and thorough (with FIT, DTB/Kernel and initramfs authentication).

It should be possible to use this from SPL, if you can enable FIT in SPL. The current implementation does not support verification, and is deliberately cut down. See common/spl/spl_fit.c.

Oh, I just noticed this file after doing a pull, that's really one step ahead of the U-Boot versions I've worked with so far. Upon further digging I found that the general SPL FIT approach is actually something we are trying to enable for our own customers moving forward. So adding/ enabling FIT auth in SPL would really help connecting the dots and closing the current authentication gap not just for us but actually for all U-Boot users.

Will look at this more closely and see how much overhead this would involve since for SPL memory can be of an issue, as using SPL authenticated FIT will probably mean pulling in the U-Boot crypto stuff as in such case we would be using U-Boot tooling to generate the signed FIT image (as opposed to a vendor-specific signing tool generating an image compatible with a simple SoC ROM API auth call). But looking at the already memory-optimized U-Boot RSA verification code in rsa-verify.c and rsa-checksum.c I would hope the impact would not be too bad. I'd guess maybe 10-20KB total with SHA256, RSA, and the needed code changes to spl_fit.c.

Quoted 3 lines

but you could perhaps
provide an option to use the full U-Boot implementation instead.

...which would mean that the entire U-Boot would need to be loaded initially as one piece which wouldn't work on some of our SoCs due to memory constraints (hence the SPL approach).

--
Andreas Dannenberg
Texas Instruments Inc

Hi Andreas,

Quoted 46 lines · Andreas Dannenberg

Hi Simon, thanks for the feedback. Additional comments inlined...

Quoted 33 lines · Simon Glass

Hi Andreas,

Quoted 30 lines · Andreas Dannenberg

Quoted 3 lines · Tom Rini

I'm interested in getting secure device support going, but it seems like we should need more than that, ie something to keep the chain of trust going.

Tom et al.,
I just saw your reply to Vitaly's email and I'm actually just looking into something along the lines you brought up but I didn't want to hijack that discussion so here's a new thread.

As for the chain of trust for ARMv7, my understanding is that when using a combination of SPL and U-Boot there will always be a vendor- specific initial boot (ROM) code that authenticates SPL, and then there will need to be some code inserted into SPL that authenticates U-Boot after it's loaded (for example by using some secure ROM API call and such).

So I was looking into if there is already some generic framework for this in U-Boot but didn't see anything obvious. One "easy" way would be to add a simple call to an authentication routine to board_init_r (u-boot/common/spl/spl.c) but let's say we add such a call for TI or other vendor's stuff I suppose this would not scale very well.

But what about adding one generic call to a default authentication function declared as __weak for spl_image that doesn't do anything, but can be overwritten in vendor-specific files to provide means of authenticating spl_image. Would this be a good approach?

Quoted 8 lines

Quoted 4 lines

Beyond that I was reviewing some of the awesome work from the Chromium team and I think on ARMv7 after we get MLO to authenticate U-Boot everything beyond that is already looking very solid and thorough (with FIT, DTB/Kernel and initramfs authentication).

It should be possible to use this from SPL, if you can enable FIT in SPL. The current implementation does not support verification, and is deliberately cut down. See common/spl/spl_fit.c.

Quoted 7 lines

Oh, I just noticed this file after doing a pull, that's really one step ahead of the U-Boot versions I've worked with so far. Upon further digging I found that the general SPL FIT approach is actually something we are trying to enable for our own customers moving forward. So adding/ enabling FIT auth in SPL would really help connecting the dots and closing the current authentication gap not just for us but actually for all U-Boot users.

Agreed.

Quoted 11 lines

Will look at this more closely and see how much overhead this would involve since for SPL memory can be of an issue, as using SPL authenticated FIT will probably mean pulling in the U-Boot crypto stuff as in such case we would be using U-Boot tooling to generate the signed FIT image (as opposed to a vendor-specific signing tool generating an image compatible with a simple SoC ROM API auth call). But looking at the already memory-optimized U-Boot RSA verification code in rsa-verify.c and rsa-checksum.c I would hope the impact would not be too bad. I'd guess maybe 10-20KB total with SHA256, RSA, and the needed code changes to spl_fit.c.

The overhead for U-Boot itself is covered in the 'Verified Boot on Chrome OS and How to do it yourself' talk here:

http://elinux.org/ELC_Europe_2013_Presentations

Verified boot itself is about 6KB on Thumb 2, on top of the FIT overhead.

Quoted 6 lines

Quoted 2 lines

but you could perhaps
provide an option to use the full U-Boot implementation instead.

...which would mean that the entire U-Boot would need to be loaded initially as one piece which wouldn't work on some of our SoCs due to memory constraints (hence the SPL approach).

No I don't mean that. I mean use the full U-Boot FIT implementation, i.e. just the same code. It would still run in FIT.

Much of the bloat
is messages which can be dropped. I did an experiment on this a while ago and already have it on my list to see if I can find any remnants of it.

Quoted 71 lines · Simon Glass

Hi Andreas,

Quoted 46 lines · Andreas Dannenberg

Hi Simon, thanks for the feedback. Additional comments inlined...

Quoted 33 lines · Simon Glass

Hi Andreas,

Quoted 30 lines · Andreas Dannenberg

Quoted 3 lines · Tom Rini

I'm interested in getting secure device support going, but it seems like we should need more than that, ie something to keep the chain of trust going.

Tom et al.,
I just saw your reply to Vitaly's email and I'm actually just looking into something along the lines you brought up but I didn't want to hijack that discussion so here's a new thread.

As for the chain of trust for ARMv7, my understanding is that when using a combination of SPL and U-Boot there will always be a vendor- specific initial boot (ROM) code that authenticates SPL, and then there will need to be some code inserted into SPL that authenticates U-Boot after it's loaded (for example by using some secure ROM API call and such).

So I was looking into if there is already some generic framework for this in U-Boot but didn't see anything obvious. One "easy" way would be to add a simple call to an authentication routine to board_init_r (u-boot/common/spl/spl.c) but let's say we add such a call for TI or other vendor's stuff I suppose this would not scale very well.

But what about adding one generic call to a default authentication function declared as __weak for spl_image that doesn't do anything, but can be overwritten in vendor-specific files to provide means of authenticating spl_image. Would this be a good approach?

Quoted 8 lines

Quoted 4 lines

Beyond that I was reviewing some of the awesome work from the Chromium team and I think on ARMv7 after we get MLO to authenticate U-Boot everything beyond that is already looking very solid and thorough (with FIT, DTB/Kernel and initramfs authentication).

It should be possible to use this from SPL, if you can enable FIT in SPL. The current implementation does not support verification, and is deliberately cut down. See common/spl/spl_fit.c.

Quoted 7 lines

Oh, I just noticed this file after doing a pull, that's really one step ahead of the U-Boot versions I've worked with so far. Upon further digging I found that the general SPL FIT approach is actually something we are trying to enable for our own customers moving forward. So adding/ enabling FIT auth in SPL would really help connecting the dots and closing the current authentication gap not just for us but actually for all U-Boot users.

Agreed.

Quoted 11 lines

Will look at this more closely and see how much overhead this would involve since for SPL memory can be of an issue, as using SPL authenticated FIT will probably mean pulling in the U-Boot crypto stuff as in such case we would be using U-Boot tooling to generate the signed FIT image (as opposed to a vendor-specific signing tool generating an image compatible with a simple SoC ROM API auth call). But looking at the already memory-optimized U-Boot RSA verification code in rsa-verify.c and rsa-checksum.c I would hope the impact would not be too bad. I'd guess maybe 10-20KB total with SHA256, RSA, and the needed code changes to spl_fit.c.

Quoted 6 lines

The overhead for U-Boot itself is covered in the 'Verified Boot on Chrome OS and How to do it yourself' talk here:

http://elinux.org/ELC_Europe_2013_Presentations

Verified boot itself is about 6KB on Thumb 2, on top of the FIT overhead.

Hi Simon,
I had actually seen/reviewed this presentation earlier and thought it was very helpful to get started so thanks for pointing this out again.

Quoted 9 lines

Quoted 6 lines

Quoted 2 lines

but you could perhaps
provide an option to use the full U-Boot implementation instead.

...which would mean that the entire U-Boot would need to be loaded initially as one piece which wouldn't work on some of our SoCs due to memory constraints (hence the SPL approach).

No I don't mean that. I mean use the full U-Boot FIT implementation, i.e. just the same code. It would still run in FIT.

Ok understood, that would seem to make it easier/cleaner from an implementation POV. Still, I've concerns specifically with one of our SoCs of which its high-security device variant apparently only has something to the order of 45KB for SPL use, which folks had to make a lot of effort to strip down/squeeze in a custom SPL, even going as far as trimming down strings (and that's using a proprietary authentication scheme for U-Boot that almost doesn't take any space at all). I need to find some time to dig deeper and play with the bits and bytes but my current thinking is to try to pursue the SPL-based FIT approach you suggested for the general use case, but have one exception for that particular one SoC using a vendor-specific authentication approach since there might not be another way due to the memory constraints.

Quoted 4 lines

Much of the bloat
is messages which can be dropped. I did an experiment on this a while ago and already have it on my list to see if I can find any remnants of it.

Yeah please let me/us know if there is something you have that could be recycled.

Hi,

Quoted 2 lines · Andreas Dannenberg

Yeah please let me/us know if there is something you have that could be recycled.

I just sent a patch referring to this. I didn't find anything but in fact it is pretty easy now as a lot of the work is done. Or at least he hard part is now the logic of which image to load, not the code building.

http://patchwork.ozlabs.org/patch/617228/

Also see Teddy's version:

http://patchwork.ozlabs.org/patch/617229/