On 29/11/19 19:20, Dr. David Alan Gilbert wrote: > * Paolo Bonzini (pbonz...@redhat.com) wrote: >> On 29/11/19 19:01, Dr. David Alan Gilbert wrote: >>>> It's not entirely trivial because fsdev-proxy-helper wants to keep the >>>> effective set and clear the permitted set; in libcap-ng you can only >> ^^^^^ >> >> (Wrong, this is "modify" the permitted set. The permitted set is >> already cleared by setresuid/setresgid). >> >>>> apply both sets at once, and you cannot choose only one of them in >>>> capng_clear/capng_get_caps_process. But it's doable, I'll take a look. >>> I'm having some difficulties making the same conversion for virtiofsd; >>> all it wants to do is drop (and later recover) CAP_FSETID >>> from it's effective set; so I'm calling capng_get_caps_process >>> (it used to be cap_get_proc). While libcap survives just using the >>> capget syscall, libcap-ng wants to read /proc/<TID>/status - and >>> that's a problem because we're in a sandbox without /proc mounted >>> at that point. >> >> The state of libcap-ng persists after capng_apply. So you can just call >> capng_update({CAP_ADD,CAP_DROP}) followed by capng_apply. > > But the internal state needs initialising doesn't it? So that when you > capng_update it tweaks a set that was originally read from somewhere? > (and that's per-thread?)
Yes, it's per thread. The state can be built from capng_clear/capng_get_caps_process + capng_update, and left in there forever. There is also capng_save_state/capng_restore_state which, as far as I can see from the sources, can be used across threads. >> Does virtiofsd have to do uid/gid dances like virtfs-proxy-helper? > > It looks like it; I can see setresuid calls to save and restore > euid/egid. Ok, then perhaps you can take a look at my virtfs-proxy-helper patch. The important part is that after setresuid/setresgid PERM=EFF if uid=0/gid=0 and PERM=0 otherwise. Paolo