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

[Jann Horn]

On Thu, Oct 6, 2022 at 5:25 PM Kees Cook <keescook at chromium.org> 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...

Random idea that I haven't thought about a lot:

One approach might be to not do it by counting, but instead have a
flag on the fs_struct that we set when someone does a clone() with
CLONE_FS but without CLONE_THREAD? Then we'd end up with the following
possible states for fs_struct:

 - single-process, normal
 - single-process, pending execve past check_unsafe_exec() (prevent
concurrent CLONE_FS)
 - shared between processes

The slight difference from the old semantics would be that once you've
used CLONE_FS without CLONE_THREAD, you can never do setuid execve()
from your current process again (without calling unshare()), even if
the child disappears in the meantime. I think that might be an
acceptably small UAPI break.