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

[David Laight]

From: Christian Brauner
> Sent: 06 October 2022 10:05
> 
> 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.

The thought of that is entirely horrid...

I presume a suid exec will fail in that case?

If the old code is trying to compare the number of threads
with the number of users of the fs table isn't is just buggy?
If a thread unshares the fs table there can be another
reference somewhere else - which is what (I presume) is being
tested for.

	David

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