Re: [RFC PATCH 4/7] kconfig: support new special property shell=

[Masahiro Yamada]

2018-02-13 0:46 GMT+09:00 Kees Cook <keescook@xxxxxxxxxxxx>:
> On Mon, Feb 12, 2018 at 2:44 AM, Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>> Linus said:
>>
>>> But yes, I also reacted to your earlier " It can't silently rewrite it
>>> to _REGULAR because the compiler support for _STRONG regressed."
>>> Because it damn well can. If the compiler doesn't support
>>> -fstack-protector-strong, we can just fall back on -fstack-protector.
>>> Silently. No extra crazy complex logic for that either.
>>
>>
>> If I understood his comment correctly,
>> we do not need either WANT_* or _AUTO.
>>
>>
>> Kees' comment:
>>
>>> In the stack-protector case, this becomes quite important, since the
>>> goal is to record the user's selection regardless of compiler
>>> capability. For example, if someone selects _REGULAR, it shouldn't
>>> "upgrade" to _STRONG. (Similarly for _NONE.)
>>
>> No.  Kconfig will not do this silently.
>>
>> "make oldconfig" (or "make silentoldconfig" as well)
>> always ask users about new symbols.
>
> The case I want to make sure can never happen is to have a config
> setting that ends up not actually being true. For example, if
> CONFIG_CC_STACKPROTECTOR appears in /proc/config.gz but the kernel
> wasn't actually built with a stack protector, that's bad. We end up in
> a place where the user can't trust the config to represent the actual
> results of the build.
>
> So, as long as the Kconfig options are strongly tied to the compiler
> capabilities, we're good on that front.
>
>> So, I can suggest to remove _REGULAR and _NONE.
>>
>> We have just two bool options, like follows.
>>
>> ------------------->8-----------------
>> config CC_STACKPROTECTOR
>>         bool "Use stack protector"
>>         depends on CC_HAS_STACKPROTECTOR
>>
>> config CC_STACKPROTECTOR_STRONG
>>         bool "Use strong strong stack protector"
>>         depends on CC_STACKPROTECTOR
>>         depends on CC_HAS_STACKPROTECTOR_STRONG
>> -------------------->8------------------
>>
>> This will work well for all{yes,mod,no}config.
>
> This two-option arrangement is fine (though it assumes there won't be
> another stack protector option in the future).
>
> The issue I have is this removes _AUTO, which I think can be solved in
> the two-option arrangement too. The purpose of _AUTO is to effectively
> enable stack-protector by default. As this option has been available
> for over 10 years, and all distros enable it, it's an obvious
> candidate to be enabled-by-default, especially since it kills a class
> of exploits (as mentioned in my commit log: BlueBorne was trivially
> defeated with stack-protector). So it should be possible to make these
> two options "default y", yes?


Yes.

Both should be "default y" to keep the equivalent behavior
since the current default is _AUTO.




>> We will not have a case where
>> -fstack-protector-strong is supported, but -fstack-protector is not.
>
> Correct.
>
> -Kees
>
> --
> Kees Cook
> Pixel Security
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-- 
Best Regards
Masahiro Yamada