[Bug 121073] [kernel] [patch] run chroot as an unprivileged user

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=121073

--- Comment #7 from Jille at quis.cx ---
I remember someone saying this could be exploited using rfork. I don't know why
it's not listed in this bug.

IIRC the problem was that fd_rdir (root of the processes) was stored in
proc->p_fd (struct filedesc) and the P_NOSUGID-flag in struct proc itself. One
could use rfork to create a new process with the same descriptor table and call
chroot in the child which would flag the child with P_NOSUGID but change to
root for the parent as well. The parent doesn't get P_NOSUGID however and will
be able to execve a setuid executable with a fake libc.

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