[PATCH 1/2] fs/exec: Explicitly unshare fs_struct on exec

[Kees Cook]

On October 6, 2022 7:13:37 AM PDT, Jann Horn <jannh at google.com> wrote:
>On Thu, Oct 6, 2022 at 11:05 AM Christian Brauner <brauner at kernel.org> wrote:
>> On Thu, Oct 06, 2022 at 01:27:34AM -0700, Kees Cook wrote:
>> > The check_unsafe_exec() counting of n_fs would not add up under a heavily
>> > threaded process trying to perform a suid exec, causing the suid portion
>> > to fail. This counting error appears to be unneeded, but to catch any
>> > possible conditions, explicitly unshare fs_struct on exec, if it ends up
>>
>> Isn't this a potential uapi break? Afaict, before this change a call to
>> clone{3}(CLONE_FS) followed by an exec in the child would have the
>> parent and child share fs information. So if the child e.g., changes the
>> working directory post exec it would also affect the parent. But after
>> this change here this would no longer be true. So a child changing a
>> workding directoro would not affect the parent anymore. IOW, an exec is
>> accompanied by an unshare(CLONE_FS). Might still be worth trying ofc but
>> it seems like a non-trivial uapi change but there might be few users
>> that do clone{3}(CLONE_FS) followed by an exec.
>
>I believe the following code in Chromium explicitly relies on this
>behavior, but I'm not sure whether this code is in active use anymore:
>
>https://source.chromium.org/chromium/chromium/src/+/main:sandbox/linux/suid/sandbox.c;l=101?q=CLONE_FS&sq=&ss=chromium

Oh yes. I think I had tried to forget this existed. Ugh. Okay, so back to the drawing board, I guess. The counting will need to be fixed...

It's possible we can move the counting after dethread -- it seems the early count was just to avoid setting flags after the point of no return, but it's not an error condition...

-- 
Kees Cook