[U-Boot] Verified boot and Legacy Kernel Images

[U-Boot] Verified boot and Legacy Kernel Images

[Mike Pearce]

Please help as I am confused.

I implemented verified boot on 2014.04 using CONFIG_OF_SEPARATE and it
works fine with FIT images. However it still boots the resident legacy
kernal that has not been signed.

This means that anyone wishing to circumvent the signed hash can do so by
replacing the image file with a legacy one. That makes for a security hole
and so I must have done something wrong.

When I look at function bootm_find_os() from file cmd_bootm.c its switch
statement provides this behaviour -

 case IMAGE_FORMAT_LEGACY:
        cool, its a go from me. Verify using an unsigned hash.
        break;
#if defined(CONFIG_FIT)
  case IMAGE_FORMAT_FIT:
        do the signed hash checks when loading the image.
        break;

What I cannot find in the code is anything that I can compile in to prevent
an unsigned legacy kernel from booting. The defines I already used include

  #define CONFIG_OF_LIBFDT
  #define CONFIG_CMD_HASH
  #define CONFIG_HASH_VERIFY
  #define CONFIG_FIT_SIGNATURE
  #define CONFIG_RSA

Thanks,

Mike.

[U-Boot] Verified boot and Legacy Kernel Images

[Heiko Schocher]

Hello Mike,

Am 05.05.2014 16:27, schrieb Mike Pearce:
> Please help as I am confused.
>
> I implemented verified boot on 2014.04 using CONFIG_OF_SEPARATE and it
> works fine with FIT images. However it still boots the resident legacy
> kernal that has not been signed.
>
> This means that anyone wishing to circumvent the signed hash can do so by
> replacing the image file with a legacy one. That makes for a security hole
> and so I must have done something wrong.

No, you did nothing wrong ...

> When I look at function bootm_find_os() from file cmd_bootm.c its switch
> statement provides this behaviour -
>
>   case IMAGE_FORMAT_LEGACY:
>          cool, its a go from me. Verify using an unsigned hash.
>          break;
> #if defined(CONFIG_FIT)
>    case IMAGE_FORMAT_FIT:
>          do the signed hash checks when loading the image.
>          break;
>
> What I cannot find in the code is anything that I can compile in to prevent
> an unsigned legacy kernel from booting. The defines I already used include
>
>    #define CONFIG_OF_LIBFDT
>    #define CONFIG_CMD_HASH
>    #define CONFIG_HASH_VERIFY
>    #define CONFIG_FIT_SIGNATURE
>    #define CONFIG_RSA

See this thread:

http://lists.denx.de/pipermail/u-boot/2014-May/178800.html

in particular Simons statement:
http://lists.denx.de/pipermail/u-boot/2014-May/178922.html

-> Currently, nothing prevents to boot an unsigned legacy kernel ...

Patches are welcome ;-)

bye,
Heiko
-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

[U-Boot] Verified boot and Legacy Kernel Images

[Simon Glass]

Hi,

On 6 May 2014 00:38, Heiko Schocher <hs@denx.de> wrote:
> Hello Mike,
>
> Am 05.05.2014 16:27, schrieb Mike Pearce:
>>
>> Please help as I am confused.
>>
>> I implemented verified boot on 2014.04 using CONFIG_OF_SEPARATE and it
>> works fine with FIT images. However it still boots the resident legacy
>> kernal that has not been signed.
>>
>> This means that anyone wishing to circumvent the signed hash can do so by
>> replacing the image file with a legacy one. That makes for a security hole
>> and so I must have done something wrong.
>
>
> No, you did nothing wrong ...
>
>> When I look at function bootm_find_os() from file cmd_bootm.c its switch
>> statement provides this behaviour -
>>
>>   case IMAGE_FORMAT_LEGACY:
>>          cool, its a go from me. Verify using an unsigned hash.
>>          break;
>> #if defined(CONFIG_FIT)
>>    case IMAGE_FORMAT_FIT:
>>          do the signed hash checks when loading the image.
>>          break;
>>
>> What I cannot find in the code is anything that I can compile in to
>> prevent
>> an unsigned legacy kernel from booting. The defines I already used include
>>
>>    #define CONFIG_OF_LIBFDT
>>    #define CONFIG_CMD_HASH
>>    #define CONFIG_HASH_VERIFY
>>    #define CONFIG_FIT_SIGNATURE
>>    #define CONFIG_RSA
>
>
> See this thread:
>
> http://lists.denx.de/pipermail/u-boot/2014-May/178800.html
>
> in particular Simons statement:
> http://lists.denx.de/pipermail/u-boot/2014-May/178922.html
>
> -> Currently, nothing prevents to boot an unsigned legacy kernel ...
>
> Patches are welcome ;-)

To close the loop, Heiko's patch (commit 21d29f7f) to fix this was
merged in May. The new default behaviour is to disable legacy format
unless CONFIG_IMAGE_FORMAT_LEGACY is defined. So this should fix the
problem.

Note also the -r flag to mkimage which marks a key as 'required to be verified'

Regards,
Simon

[U-Boot] Verified boot and Legacy Kernel Images

[mike]

Hi Simon,

Thanks and to Heiko also.

Mike
On 09/05/2014 11:54 PM, Simon Glass wrote:
> Hi,
>
> On 6 May 2014 00:38, Heiko Schocher <hs@denx.de> wrote:
>> Hello Mike,
>>
>> Am 05.05.2014 16:27, schrieb Mike Pearce:
>>> Please help as I am confused.
>>>
>>> I implemented verified boot on 2014.04 using CONFIG_OF_SEPARATE and it
>>> works fine with FIT images. However it still boots the resident legacy
>>> kernal that has not been signed.
>>>
>>> This means that anyone wishing to circumvent the signed hash can do so by
>>> replacing the image file with a legacy one. That makes for a security hole
>>> and so I must have done something wrong.
>>
>> No, you did nothing wrong ...
>>
>>> When I look at function bootm_find_os() from file cmd_bootm.c its switch
>>> statement provides this behaviour -
>>>
>>>    case IMAGE_FORMAT_LEGACY:
>>>           cool, its a go from me. Verify using an unsigned hash.
>>>           break;
>>> #if defined(CONFIG_FIT)
>>>     case IMAGE_FORMAT_FIT:
>>>           do the signed hash checks when loading the image.
>>>           break;
>>>
>>> What I cannot find in the code is anything that I can compile in to
>>> prevent
>>> an unsigned legacy kernel from booting. The defines I already used include
>>>
>>>     #define CONFIG_OF_LIBFDT
>>>     #define CONFIG_CMD_HASH
>>>     #define CONFIG_HASH_VERIFY
>>>     #define CONFIG_FIT_SIGNATURE
>>>     #define CONFIG_RSA
>>
>> See this thread:
>>
>> http://lists.denx.de/pipermail/u-boot/2014-May/178800.html
>>
>> in particular Simons statement:
>> http://lists.denx.de/pipermail/u-boot/2014-May/178922.html
>>
>> -> Currently, nothing prevents to boot an unsigned legacy kernel ...
>>
>> Patches are welcome ;-)
> To close the loop, Heiko's patch (commit 21d29f7f) to fix this was
> merged in May. The new default behaviour is to disable legacy format
> unless CONFIG_IMAGE_FORMAT_LEGACY is defined. So this should fix the
> problem.
>
> Note also the -r flag to mkimage which marks a key as 'required to be verified'
>
> Regards,
> Simon
>