[PATCH v4 2/5] userns: don't let setns() lend CAP_SETFCAP

[sashiko-bot at kernel.org]

> Mapping uid 0 of the parent into a user namespace requires CAP_SETFCAP,
> because a task in such a namespace can write file capabilities that are
> honoured in the parent.  A task that is already in the namespace has a
> full capability set there and can't be asked any more, so
> verify_root_map() goes by what the creator had, now ns->setfcap_level.
> 
> But setns() lets other tasks in, and they get the full set as well:
> 
>   task A: uid 0, full caps          task B: uid 0, no CAP_SETFCAP
>   unshare(CLONE_NEWUSER)
>                                     setns(A's user ns)
>                                       allowed, euid == ns->owner
>                                     write "0 0 1" to /proc/self/uid_map
>                                       ns->setfcap_level is A's -> allowed
> 
> [ ... ]
> 
> Link: https://lore.kernel.org/all/20261021-bioenergie-allheilmittel-undurchdringlichen-35666eb62cff@brauner/
> Fixes: db2e718a4798 ("capabilities: require CAP_SETFCAP to map uid 0")
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef at toxicpanda.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261022-b4-setfcap-userns-v4-0-88aa0ffaa08f@toxicpanda.com?part=2