[PATCH] dfu: Prevent set_dfu_alt_info() from overwriting a previous value
Jon Humphreys
j-humphreys at ti.com
Mon Feb 3 22:38:35 CET 2025
Ilias Apalodimas <ilias.apalodimas at linaro.org> writes:
> Hi Jon,
>
> On Fri, 17 Jan 2025 at 00:02, Jon Humphreys <j-humphreys at ti.com> wrote:
>>
>> Sughosh Ganu <sughosh.ganu at linaro.org> writes:
>>
>> > On Thu, 16 Jan 2025 at 14:07, Mattijs Korpershoek
>> > <mkorpershoek at baylibre.com> wrote:
>> >>
>> >> Hi Jon,
>> >>
>> >> Sorry for the (very) late reply. I had some long holidays in between and
>> >> since this is a difficult topic for me, I kept pushing this to the end
>> >> of my backlog.
>> >>
>> >> On mer., déc. 18, 2024 at 17:09, Jon Humphreys <j-humphreys at ti.com> wrote:
>> >>
>> >> > Mattijs Korpershoek <mkorpershoek at baylibre.com> writes:
>> >> >
>> >> >> Hi Jonathan,
>> >> >>
>> >> >> Thank you for the patch.
>> >> >>
>> >> >> On mar., déc. 17, 2024 at 14:48, Jonathan Humphreys <j-humphreys at ti.com> wrote:
>> >> >>
>> >> >>> If CONFIG_SET_DFU_ALT_INFO is enabled, the dfu_alt_info environment
>> >> >>> variable is dynamically set when initializing the DFU entities, which is
>> >> >>> done as part of normal flow of a DFU operation.
>> >> >>>
>> >> >>> The USB DFU boot support will set it's own specific value for dfu_alt_info
>> >> >>> before performing the DFU operation. This means that if
>> >> >>> CONFIG_SET_DFU_ALT_INFO is enabled, the dfu_alt_info environment variable
>> >> >>> that the USB DFU boot path had set is overwritten, causing USB DFU boot to
>> >> >>> fail.
>> >> >>>
>> >> >>> Likewise, if the user sets their own value for dfu_alt_info, say at the
>> >> >>> U-Boot prompt, it get's overwritten if CONFIG_SET_DFU_ALT_INFO is enabled.
>> >> >>>
>> >> >>> This patch will first check that dfu_alt_info isn't already set before
>> >> >>> calling set_dfu_alt_info(), when CONFIG_SET_DFU_ALT_INFO is enabled.
>> >> >>
>> >> >> To me, this is a policy change: before we could override the environment
>> >> >> via set_dfu_alt_info(). Now we cannot anymore (if "dfu_alt_info" is already
>> >> >> set in the environment).
>> >> >>
>> >> >> Also, it seems that this change goes against the uefi doc which states:
>> >> >>
>> >> >> """
>> >> >> A string is defined which is to be used for populating the
>> >> >> dfu_alt_info variable. This string is used by the function
>> >> >> set_dfu_alt_info. Instead of taking the variable from the environment,
>> >> >> the capsule update feature requires that the variable be set through
>> >> >> the function, since that is more robust. Allowing the user to change
>> >> >> the location of the firmware updates is not a very secure
>> >> >> practice. Getting this information from the firmware itself is more
>> >> >> secure, assuming the firmware has been verified by a previous stage
>> >> >> boot loader.
>> >> >> """
>> >> >>
>> >> >> See: https://docs.u-boot.org/en/latest/develop/uefi/uefi.html#performing-the-update
>> >> >>
>> >> >> Moreover, looking at various boards that implement
>> >> >> set_dfu_alt_info(), we can see different behaviours:
>> >> >>
>> >> >> Boards examples that won't override "dfu_alt_info" via
>> >> >> set_dfu_alt_info() if "dfu_alt_info" is already set via environment
>> >> >>
>> >> >> * board/xilinx/zynq/board.c
>> >> >> * board/emulation/common/qemu_dfu.c
>> >> >>
>> >> >> Boards examplesthat will always override the "dfu_alt_info" via
>> >> >> set_dfu_alt_info():
>> >> >>
>> >> >> * board/libre-computer/aml-a311d-cc/aml-a311d-cc.c
>> >> >> * board/ti/am62px/evm.c
>> >> >>
>> >> >> Since set_dfu_alt_info() is a board specific callback, why can't this
>> >> >> logic be implemented for boards that want this behaviour change?
>> >> >
>> >> > Because I would then need to duplicate the same logic for every board that
>> >> > wanted both USB DFU boot and EFI capsules to work. And the paramters
>> >> > passed in do not allow the function to know the use case (am I DFU booting
>> >> > or updating EFI capsules?). See more below.
>> >>
>> >> I understand that duplicating logic for every board you maintain is not
>> >> optimal, however, it gives each vendor the freedom of implementing their
>> >> policy.
>> >>
>> >> I've added a couple of folks who I think could help giving their opinion on EFI capsules/policy.
>> >>
>> >> Heinrich, Ilias, Sugosh, do you have any opinion on this patch?
>> >>
>> >> >
>> >> >>
>> >> >> Regards,
>> >> >>
>> >> >> Mattijs
>> >> >>
>> >> >>>
>> >> >>> Signed-off-by: Jonathan Humphreys <j-humphreys at ti.com>
>> >> >>> ---
>> >> >>> drivers/dfu/dfu.c | 7 +++++--
>> >> >>> 1 file changed, 5 insertions(+), 2 deletions(-)
>> >> >>>
>> >> >>> diff --git a/drivers/dfu/dfu.c b/drivers/dfu/dfu.c
>> >> >>> index 756569217bb..ab8abae1d89 100644
>> >> >>> --- a/drivers/dfu/dfu.c
>> >> >>> +++ b/drivers/dfu/dfu.c
>> >> >>> @@ -169,10 +169,13 @@ int dfu_init_env_entities(char *interface, char *devstr)
>> >> >>> dfu_reinit_needed = false;
>> >> >>> dfu_alt_info_changed = false;
>> >> >>>
>> >> >>> + str_env = env_get("dfu_alt_info");
>> >> >>> #ifdef CONFIG_SET_DFU_ALT_INFO
>> >> >>> - set_dfu_alt_info(interface, devstr);
>> >> >>> + if (!str_env) {
>> >> >>> + set_dfu_alt_info(interface, devstr);
>> >> >>> + str_env = env_get("dfu_alt_info");
>> >> >>> + }
>> >> >>> #endif
>> >> >>> - str_env = env_get("dfu_alt_info");
>> >> >>> if (!str_env) {
>> >> >>> pr_err("\"dfu_alt_info\" env variable not defined!\n");
>> >> >>> return -EINVAL;
>> >> >>> --
>> >> >>> 2.34.1
>> >> >
>> >> > Mattijs, thanks for the thorough reply. I did wrestle a lot with how wide
>> >> > of a fix to propose for this problem, and in the end, decided on the narrow
>> >> > fix of simply preventing the overwriting of the variable.
>> >> >
>> >> > Yes it is a policy change, but the policy is already unclear, inconsistent,
>> >> > and confusing, IMO.
>> >> >
>> >> > For example:
>> >> > 1) EFI capsule update wants to very strictly control the dfu alt values
>> >> > by setting it in set_dfu_alt_info(), but then any other DFU use
>> >> > case breaks. USB DFU boot is now broken.
>> >> > 2) The behavior the user sees wrt the dfu_alt_info env variable is very
>> >> > confusing and non-intuitive. Take this example:
>> >> >
>> >> > => env set dfu_alt_info "sf 0:0=exe1.bin raw 0 88000;exe2.bin raw 88000 100000"
>> >> > => env print dfu_alt_info
>> >> > dfu_alt_info=sf 0:0=exe1.bin raw 0 88000;exe2.bin raw 88000 100000
>> >> > => dfu 0 list
>> >> > DFU alt settings list:
>> >> > dev: SF alt: 0 name: tiboot3.bin layout: RAW_ADDR
>> >> > dev: SF alt: 1 name: tispl.bin layout: RAW_ADDR
>> >> > dev: SF alt: 2 name: u-boot.img layout: RAW_ADDR
>> >> > => env print dfu_alt_info
>> >> > dfu_alt_info=sf 0:0=tiboot3.bin raw 0 80000;tispl.bin raw 80000 200000;u-boot.img raw 280000 400000
>> >> > =>
>> >> >
>> >> > As you can see, the user set's the dfu_alt_info variable according to their
>> >> > specific use case, then simply tries to list the DFU alt settings, and
>> >> > because this code goes through the dfu_init_env_entities() path, it gets
>> >> > changed to the EFI capsule settings.
>> >> >
>> >> > I was hoping to get a simpler fix in now so we can get USB DFU boot working
>> >> > again, and we can visit the overall policy design next. As you suggest, I
>> >> > could also push the testing of overwriting into the board specific
>> >> > set_dfu_alt_info() function, but then I need to duplicate the code in 8
>> >> > different places for the TI boards, and other vendors may still have the
>> >> > problem.
>> >>
>> >> I agree that the above behaviour is confusing and I'm reconsidering to
>> >> take up this patch. I'd like some buy-in from either Heinrich, Ilias or
>> >> Sughosh on this since I'm not 100% confortable with the "policy change"
>> >
>> > A little context here. The DFU driver already had this policy in place
>> > where, if the CONFIG_SET_DFU_ALT_INFO was set, the dfu_alt_info string
>> > would be set by U-Boot, instead of taking the user provided string. It
>> > was decided to use this for EFI capsule updates, as getting the string
>> > which determines the location of writing the update images from within
>> > U-Boot is more resilient than taking some user provided string.
>> >
>> >
>> >>
>> >> >
>> >> > Looking to the longer term solution, here are my thoughts.
>> >> > 1) We need to decouple CONFIG_SET_DFU_ALT_INFO from EFI capsules. The only
>> >> > reason TI boards are now setting CONFIG_SET_DFU_ALT_INFO is because EFI
>> >> > capsule update is enabled. Outside of a few legacy uses (I think - it
>> >> > appears they were introduced prior to supporting multi-interface dfu alt
>> >> > strings), I think this is true for other vendor's boards as well.
>> >> > 2) Have EFI capsule support do as USB DFU boot does today, and set the
>> >> > dfu alt string it wants used *before* initiating the DFU operation. With
>> >> > CONFIG_SET_DFU_ALT_INFO no longer enabled, the value it set will not get
>> >> > overridden.
>> >> > 3) Have the actual value of the dfu alt string used in the DFU operation be
>> >> > passed in, rather than read from the dfu_alt_info environment variable.
>> >> > The USB DFU and EFI capsule use case will pass in the dfu alt string
>> >> > they want. The standard 'dfu' command can pass in the value of the
>> >> > dfu_alt_info env variable. Note that this effectively decouples the dfu
>> >> > command from the alt settings that USB DFU boot and EFI capsules use,
>> >> > but I think this is what we want.
>> >
>> > I think either of 2) or 3) above can be looked at. Although not sure
>> > if 2) will be breaking the current DFU policy.
>> >
>> > -sughosh
>> >
>>
>> Thanks for the comments. I have looked into this a bit further and see 2
>> options we can take for the EFI capsule update use case:
>>
>> 1) stick with the more traditional approach and do as DFU BOOT does by
>> setting the value of the dfu_alt_info env variable just before
>> initiating the DFU operation. As Ilias sugggested, we could also add a
>> save/restore so this is transparent to other DFU users.
>>
>> 2) move to a model where we explicitly pass in to the DFU operation the
>> value of dfu_alt_info that we want used. In the normal/legacy DFU use
>> cases, this would involve calling env_get() for the dfu_alt_info env
>> variable and pass that value. I like this approach because it is
>> cleaner and more explicit. However, there are many layers of function
>> calls between the driver of the DFU operation (the one that would decide
>> what dfu_alt_info value to use) and dfu_init_env_entities() where it is
>> used, and passing this info across the function interfaces would
>> involved lots of function interface updates.
>>
>> I'm curious if there is apetite for 2) or should we go with the more
>> traditional approach in 1).
>>
>> The above assumes we decouple CONFIG_SET_DFU_ALT_INFO from EFI capsules.
>> For platforms still setting CONFIG_SET_DFU_ALT_INFO, the presumption is that
>> if they override the dfu_alt_info env variable, they know what they are
>> doing.
>
> Decoupling that should be pretty easy. I personally like 2) more since
> it's much more scalable and doesn't involve saving.restoring values.
> But I am not sure how big of a task it is
>
I went with option 1 above. I started working on option 2 however the
changes are quite invasive, touching all of the DFU interfaces, and I do not
have the understanding of the different DFU use cases nor access to the
different hardware using those use cases to test. I also think that this
change should be part of a broader DFU cleanup to use only the latest
dfu_alt_info string format allowing multiple devices, so that we no longer
need to pass around the interface and devstr parameters.
Please take a look at the patch series titled "EFI Capsule update
explicitly sets dfu_alt_info" that I will post shortly.
thanks
Jon
> Cheers
> /Ilias
>>
>> Thanks
>> Jon
>>
>> >> >
>> >> > This then allows both USB DFU boot and EFI capsule use cases to work as
>> >> > intended and allows the dfu command to operate on the user defined
>> >> > dfu_alt_info value.
>> >> >
>> >> > I welcome comments from those that have the history and intended behavior
>> >> > background of the DFU support :).
>> >>
>> >> I do as well. I have taken over maintaince on this subsystem a year ago
>> >> and have not had much patches/work done on the subsystem. Therefore I'm
>> >> not as knowledgeable as I would have liked to be. I'm sorry about that.
>> >>
>> >> >
>> >> > I also welcome comments on how to proceed for 2025.01. Should we live with
>> >> > USB DFU boot broken until we get the long term fix in, or ok with the patch
>> >> > posted here. The patch posted here does allow for a user to change EFI
>> >> > capsule's dfu alt settings, as Mattijs says, but especially given capsules
>> >> > can be authenticated, I'm not sure how this would be exploited, and if that
>> >> > risk is worse that broken DFU boot.
>> >> >
>> >> > thank
>> >> > Jon
More information about the U-Boot
mailing list