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

[Andrea Barisani]

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.

> 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

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