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Normally when a process' uid is changed to non-0, all the capabilities bits are cleared, even those explicitly set with calls to capng_update()/capng_apply() made immediately before setuid. And *after* the process' uid has been changed, it no longer has the necessary privileges to add capabilities back to the process. In order to set a non-0 uid while still maintaining any capabilities bits, it is necessary to either call capng_change_id() (which unfortunately doesn't currently call initgroups to setup auxiliary group membership), or to perform the small amount of calisthenics contained in the new utility function virSetUIDGIDWithCaps(). Another very important difference between the capabilities setting/clearing in virSetUIDGIDWithCaps() and virCommand's virSetCapabilities() (which it will replace in the next patch) is that the new function properly clears the capabilities bounding set, so it will not be possible for a child process to set any new capabilities. A short description of what is done by virSetUIDGIDWithCaps(): 1) clear all capabilities then set all those desired by the caller (in capBits) plus CAP_SETGID, CAP_SETUID, and CAP_SETPCAP (which is needed to change the capabilities bounding set). 2) call prctl(), telling it that we want to maintain current capabilities across an upcoming setuid(). 3) switch to the new uid/gid 4) again call prctl(), telling it we will no longer want capabilities maintained if this process does another setuid(). 5) clear the capabilities that we added to allow us to setuid/setgid/change the bounding set (unless they were also requested by the caller via the virCommand API). Because the modification/maintaining of capabilities is intermingled with setting the uid, this is necessarily done in a single function, rather than having two independent functions. Note that, due to the way that effective capabilities are computed (at time of execve) for a process that has uid != 0, the *file* capabilities of the binary being executed must also have the desired capabilities bit(s) set (see "man 7 capabilities"). This can be done with the "filecap" command. (e.g. "filecap /usr/bin/qemu-kvm sys_rawio").
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>