Re: [libvirt] [PATCH] command: avoid deadlock on EPIPE situation

It is possible to deadlock libvirt by having a domain with XML longer than PIPE_BUF, and by writing a hook script that closes stdin early. This is because libvirt was keeping a copy of the child's stdin read fd open, which means the write fd in the parent will never see EPIPE (remember, libvirt should always be run with SIGPIPE ignored, so we should never get a SIGPIPE signal). Since there is no error, libvirt blocks waiting for a write to complete, even though the only reader is also libvirt. The solution is to ensure that only the child can act as a reader before the parent does any writes; and then dealing with the fallout of dealing with EPIPE. Thankfully, this is not a security hole - since the only way to trigger the deadlock is to install a custom hook script, anyone that already has privileges to install a hook script already has privileges to do any number of other equally disruptive things to libvirt; it would only be a security hole if an unprivileged user could install a hook script to DoS a privileged user. * src/util/command.c (virCommandRun): Close parent's copy of child read fd earlier. (virCommandProcessIO): Don't let EPIPE be fatal; the child may be done parsing input. * tests/commandhelper.c (main): Set up a SIGPIPE situation. * tests/commandtest.c (test20): Trigger it. * tests/commanddata/test20.log: New file. --- I tested this by using just the changes to 'tests/', and was able to trigger deadlock in 100% of my testing with the buffer lengths shown here. src/util/command.c | 22 +++++++++++++--------- tests/commanddata/test20.log | 13 +++++++++++++ tests/commandhelper.c | 6 ++++++ tests/commandtest.c | 42 ++++++++++++++++++++++++++++++++++++++++++ 4 files changed, 74 insertions(+), 9 deletions(-) create mode 100644 tests/commanddata/test20.log