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

[Kees Cook]

On May 13, 2025 6:05:45 AM PDT, Mateusz Guzik <mjguzik at gmail.com> wrote:
>On Thu, Oct 06, 2022 at 08:25:01AM -0700, Kees Cook wrote:
>> 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...
>> 
>
>I landed here from git blame.
>
>I was looking at sanitizing shared fs vs suid handling, but the entire
>ordeal is so convoluted I'm confident the best way forward is to whack
>the problem to begin with.
>
>Per the above link, the notion of a shared fs struct across different
>processes is depended on so merely unsharing is a no-go.
>
>However, the shared state is only a problem for suid/sgid.
>
>Here is my proposal: *deny* exec of suid/sgid binaries if fs_struct is
>shared. This will have to be checked for after the execing proc becomes
>single-threaded ofc.

Unfortunately the above Chrome helper is setuid and uses CLONE_FS.

But to echo what Eric asked: what problem are you trying to solve?

-Kees

-- 
Kees Cook