Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

Madhavan T. Venkataraman
2020-08-11T15:55:03Z

On 8/11/20 8:08 AM, Pavel Machek wrote:
> Hi!
>
>>>> Thanks for the lively discussion. I have tried to answer some of the
>>>> comments below.
>>>
>>>>> There are options today, e.g.
>>>>>
>>>>> a) If the restriction is only per-alias, you can have distinct aliases
>>>>> where one is writable and another is executable, and you can make it
>>>>> hard to find the relationship between the two.
>>>>>
>>>>> b) If the restriction is only temporal, you can write instructions into
>>>>> an RW- buffer, transition the buffer to R--, verify the buffer
>>>>> contents, then transition it to --X.
>>>>>
>>>>> c) You can have two processes A and B where A generates instrucitons into
>>>>> a buffer that (only) B can execute (where B may be restricted from
>>>>> making syscalls like write, mprotect, etc).
>>>>
>>>> The general principle of the mitigation is W^X. I would argue that
>>>> the above options are violations of the W^X principle. If they are
>>>> allowed today, they must be fixed. And they will be. So, we cannot
>>>> rely on them.
>>>
>>> Would you mind describing your threat model?
>>>
>>> Because I believe you are using model different from everyone else.
>>>
>>> In particular, I don't believe b) is a problem or should be fixed.
>>
>> It is a problem because a kernel that implements W^X properly
>> will not allow it. It has no idea what has been done in userland.
>> It has no idea that the user has checked and verified the buffer
>> contents after transitioning the page to R--.
>
> No, it is not a problem. W^X is designed to protect from attackers
> doing buffer overflows, not attackers doing arbitrary syscalls.
>

Hey Pavel,

You are correct. The W^X implementation today still has some holes.
IIUC, the principle of W^X is - user should not be able to (W) write code
into a page and use some trick to get it to (X) execute. So, what I
was trying to say was that the W^X principle is not implemented
completely today.

Mark Rutland mentioned some other tricks as well which are being used
today.

For instance, Microsoft has submitted this proposal:

https://microsoft.github.io/ipe/

IPE is an LSM. In this proposal, only mappings that are backed by a
signature verified file can have execute permissions. This means that
all anonymous page based tricks will fail. And, file mapping based
tricks will fail as well when temporary files are used to load code
and mmap(). That is the intent.

Thanks!

Madhavan