A symlink-following flaw was found in libvirt's qemuTPMEmulatorPrepareHost() function (src/qemu/qemutpm.c). The function uses a path-based chown() to set ownership of the swtpm logfile without verifying that the path is not a symbolic link. Since the log directory is writable by the swtpm user, a local attacker with access to the swtpm account can replace the logfile with a symlink pointing to an arbitrary file. When libvirtd (running as root) starts a QEMU domain with an emulated TPM, it follows the symlink and transfers ownership of the target file to swtpm:swtpm, allowing the attacker to then modify that file.

The upstream fix replaces the path-based chown() with open(ONOFOLLOW) + fchown() on the file descriptor.

External reference: https://gitlab.com/libvirt/libvirt/-/workitems/909