[U-Boot] CVE-2018-18439, CVE-2018-18440 - U-Boot verified boot bypass vulnerabilities

[Simon Goldschmidt]

On Fri, Nov 9, 2018 at 10:46 AM Andrea Barisani
<andrea.barisani at f-secure.com> wrote:
>
> On Fri, Nov 09, 2018 at 07:11:36AM +0100, Simon Goldschmidt wrote:
> > On Fri, Nov 9, 2018 at 1:37 AM Fabio Estevam <festevam at gmail.com> wrote:
> > >
> > > Hi Andrea,
> > >
> > > On Tue, Nov 6, 2018 at 12:57 PM Andrea Barisani
> > > <andrea.barisani at f-secure.com> wrote:
> > >
> > > > # load large file
> > > > => ext2load mmc 0 0x60000000 fitimage.itb
> > >
> > > Does this change work for you?
> > > http://dark-code.bulix.org/u6gw3b-499924
> >
> > My understanding was U-Boot text or stack could get overwritten which
> > leads to the loaded bytes being executed as code.
> > So you would have to check that the loaded range is within ram but not
> > within that reserved range of code or stack (or heap).
> >
>
> Exactly, merely checking RAM size is not sufficient. The specific memory
> layout would need to be accounted for which means understanding where the
> stack and heap are located, their direction of growth and to ensure that the
> loaded payload can never overwrite them along with all other U-Boot data
> segments.
>
> This is not easy given that the stack and heap size I think can only be
> guessed and not precisely limited, additionally board configurations have the
> ability to set arbitrary stack, relocation and load addresses which
> complicates things even further in understanding exactly how the memory
> layout is set.

It's not easy, but in my opinion, it should already be solved by the
code in 'boot_start_lmb' mentioned in my last mail.
This function includes arch and board callbacks that should be able to
return a safe memory range.

The only thing that cannot be controlled here is stack size, that's
true. The ARM port tries to solve this by getting the current stack
pointer and subtracting "4K to be safe". As far as I know, there are
no methods in U-Boot currently to ensure this is safe, though. And
depending on the RAM size, we could just subtract more. Personally, I
wouldn't mind subtracting some MBytes on my board. Actually using such
a stack would definively be another bug that needs fixing.

But it seems a good start to use these functions to limit loading from fs, too.

Simon

>
> > Getting this reserved range is what 'boot_start_lmb' does (in
> > bootm.c). Maybe this code can be refactored and reused in fs.c to get
> > a valid range for loading?
> >
> > Additionally, your patch checks the loaded file's size without taking
> > the load address into account. So unless I read that wrong, your check
> > is only valid for 'addr == 0'.
> > Plus, the 'bytes' parameter should probably be a restriction to the
> > file's size when checking for a valid load range.
> >
> > Simon
>
> --
> Andrea Barisani     Head of Hardware Security |     F-Secure
>                                       Founder | Inverse Path
>
> https://www.f-secure.com             https://inversepath.com
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