Questions about the ownership of /var/cache/libvirt/qemu
Hi all, When we change the user for QEMU process (change "user" and "group" in /etc/libvirt/qemu.conf) to a normal user, we found that libvirtd will also change the ownership of /var/cache/libvirt/qemu but will keep the ownership of /var/cache/libvirt/qemu/capabilities as root. Is it secure to put files/directories owned by root in a directory owned by a normal user? Could the normal user replace the root's file with a new one? Does it need to set sticky bit on /var/cache/libvirt/qemu or keep the ownership of /var/cache/libvirt/qemu as root?
On 8/22/21 11:49 AM, Peng Liang wrote:
Hi all, When we change the user for QEMU process (change "user" and "group" in /etc/libvirt/qemu.conf) to a normal user, we found that libvirtd will also change the ownership of /var/cache/libvirt/qemu but will keep the ownership of /var/cache/libvirt/qemu/capabilities as root. Is it secure to put files/directories owned by root in a directory owned by a normal user? Could the normal user replace the root's file with a new one?
No, the capabilities directory lacks write perms: drwxr-xr-x 2 root root 234 Aug 20 17:11 /var/cache/libvirt/qemu/capabilities/ and caps files themselves are RW by root only: -rw------- 1 root root 144215 Aug 20 15:38 926803a9278e445ec919c2b6cbd8c1c449c75b26dcb1686b774314180376c725.xml Therefore, I don't think a regular user could spoof capabilities.
Does it need to set sticky bit on /var/cache/libvirt/qemu or keep the ownership of /var/cache/libvirt/qemu as root?
No, setting sticky bit would make caps files owned by root:group and I don't think we want that (even though, not even group can write caps files). I hope this answers your concern. Michal
On 8/23/2021 3:57 PM, Michal Prívozník wrote:
On 8/22/21 11:49 AM, Peng Liang wrote:
Hi all, When we change the user for QEMU process (change "user" and "group" in /etc/libvirt/qemu.conf) to a normal user, we found that libvirtd will also change the ownership of /var/cache/libvirt/qemu but will keep the ownership of /var/cache/libvirt/qemu/capabilities as root. Is it secure to put files/directories owned by root in a directory owned by a normal user? Could the normal user replace the root's file with a new one?
No, the capabilities directory lacks write perms:
drwxr-xr-x 2 root root 234 Aug 20 17:11 /var/cache/libvirt/qemu/capabilities/
and caps files themselves are RW by root only:
-rw------- 1 root root 144215 Aug 20 15:38 926803a9278e445ec919c2b6cbd8c1c449c75b26dcb1686b774314180376c725.xml
Therefore, I don't think a regular user could spoof capabilities.
But the regular user can rename capabilities and create a new one. For example, # runuser -u qemu -- mv /var/cache/libvirt/qemu/capabilities /var/cache/libvirt/qemu/capabilities-bak # runuser -u qemu -- mkdir /var/cache/libvirt/qemu/capabilities runuser -u qemu -- touch /var/cache/libvirt/qemu/capabilities/<hash>.xml And I found that the operations can be done even though the sticky bit is set on /var/cache/libvirt/qemu :(
Does it need to set sticky bit on /var/cache/libvirt/qemu or keep the ownership of /var/cache/libvirt/qemu as root?
No, setting sticky bit would make caps files owned by root:group and I don't think we want that (even though, not even group can write caps files).
I hope this answers your concern.
I found that libvirt will put some temporary files (e.g. screenshot files) which are used by qemu in /var/cache/libvirt/qemu. How about move those files to domain dirs in /var/run/libvirt/qemu and keep the ownership of /var/cache/libvirt/qemu as root? Thanks, Peng
Michal
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participants (2)
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Michal Prívozník -
Peng Liang