On 30/06/2020 14:35, Daniel P. Berrangé wrote: > On Tue, Jun 30, 2020 at 02:00:06PM +0200, Laurent Vivier wrote: >> On 30/06/2020 13:03, Daniel P. Berrangé wrote: >>> On Tue, Jun 30, 2020 at 12:35:46PM +0200, Laurent Vivier wrote: >>>> On 30/06/2020 12:03, Jason Wang wrote: >>>>> >>>>> On 2020/6/30 下午5:45, Laurent Vivier wrote: >>>>>> On 30/06/2020 11:31, Daniel P. Berrangé wrote: >>>>>>> On Tue, Jun 30, 2020 at 10:23:18AM +0100, Daniel P. Berrangé wrote: >>>>>>>> On Tue, Jun 30, 2020 at 05:21:49PM +0800, Jason Wang wrote: >>>>>>>>> On 2020/6/30 上午3:30, Laurent Vivier wrote: >>>>>>>>>> On 28/06/2020 08:31, Jason Wang wrote: >>>>>>>>>>> On 2020/6/25 下午7:56, Laurent Vivier wrote: >>>>>>>>>>>> On 25/06/2020 10:48, Daniel P. Berrangé wrote: >>>>>>>>>>>>> On Wed, Jun 24, 2020 at 09:00:09PM +0200, Laurent Vivier wrote: >>>>>>>>>>>>>> qemu_set_nonblock() checks that the file descriptor can be >>>>>>>>>>>>>> used and, if >>>>>>>>>>>>>> not, crashes QEMU. An assert() is used for that. The use of >>>>>>>>>>>>>> assert() is >>>>>>>>>>>>>> used to detect programming error and the coredump will allow >>>>>>>>>>>>>> to debug >>>>>>>>>>>>>> the problem. >>>>>>>>>>>>>> >>>>>>>>>>>>>> But in the case of the tap device, this assert() can be >>>>>>>>>>>>>> triggered by >>>>>>>>>>>>>> a misconfiguration by the user. At startup, it's not a real >>>>>>>>>>>>>> problem, >>>>>>>>>>>>>> but it >>>>>>>>>>>>>> can also happen during the hot-plug of a new device, and here >>>>>>>>>>>>>> it's a >>>>>>>>>>>>>> problem because we can crash a perfectly healthy system. >>>>>>>>>>>>> If the user/mgmt app is not correctly passing FDs, then there's >>>>>>>>>>>>> a whole >>>>>>>>>>>>> pile of bad stuff that can happen. Checking whether the FD is >>>>>>>>>>>>> valid is >>>>>>>>>>>>> only going to catch a small subset. eg consider if fd=9 refers >>>>>>>>>>>>> to the >>>>>>>>>>>>> FD that is associated with the root disk QEMU has open. We'll >>>>>>>>>>>>> fail to >>>>>>>>>>>>> setup the TAP device and close this FD, breaking the healthy >>>>>>>>>>>>> system >>>>>>>>>>>>> again. >>>>>>>>>>>>> >>>>>>>>>>>>> I'm not saying we can't check if the FD is valid, but lets be >>>>>>>>>>>>> clear that >>>>>>>>>>>>> this is not offering very much protection against a broken mgmt >>>>>>>>>>>>> apps >>>>>>>>>>>>> passing bad FDs. >>>>>>>>>>>>> >>>>>>>>>>>> I agree with you, but my only goal here is to avoid the crash in >>>>>>>>>>>> this >>>>>>>>>>>> particular case. >>>>>>>>>>>> >>>>>>>>>>>> The punishment should fit the crime. >>>>>>>>>>>> >>>>>>>>>>>> The user can think the netdev_del doesn't close the fd, and he >>>>>>>>>>>> can try >>>>>>>>>>>> to reuse it. Sending back an error is better than crashing his >>>>>>>>>>>> system. >>>>>>>>>>>> After that, if the system crashes, it will be for the good >>>>>>>>>>>> reasons, not >>>>>>>>>>>> because of an assert. >>>>>>>>>>> Yes. And on top of this we may try to validate the TAP via st_dev >>>>>>>>>>> through fstat[1]. >>>>>>>>>> I agree, but the problem I have is to know which major(st_dev) we can >>>>>>>>>> allow to use. >>>>>>>>>> >>>>>>>>>> Do we allow only macvtap major number? >>>>>>>>> >>>>>>>>> Macvtap and tuntap. >>>>>>>>> >>>>>>>>> >>>>>>>>>> How to know the macvtap major number at user level? >>>>>>>>>> [it is allocated dynamically: do we need to parse /proc/devices?] >>>>>>>>> >>>>>>>>> I think we can get them through fstat for /dev/net/tun and >>>>>>>>> /dev/macvtapX. >>>>>>>> Don't assume QEMU has any permission to access to these device nodes, >>>>>>>> only the pre-opened FDs it is given by libvirt. >>>>>>> Actually permissions are the least of the problem - the device nodes >>>>>>> won't even exist, because QEMU's almost certainly running in a private >>>>>>> mount namespace with a minimal /dev populated >>>>>>> >>>>>> I'm working on a solution using /proc/devices. >>>>> >>>>> >>>>> Similar issue with /dev. There's no guarantee that qemu can access >>>>> /proc/devices or it may not exist (CONFIG_PROCFS). >>>> >>>> There is a lot of things that will not work without /proc (several tools >>>> rely on /proc, like ps, top, lsof, mount, ...). Some information are >>>> only available from /proc, and if /proc is there, I think /proc/devices >>>> is always readable by everyone. Moreover /proc is already used by qemu >>>> in several places. >>>> >>>> It can also a best effort check. >>>> >>>> The problem with fstat() on /dev files is to guess the /dev/macvtapX as >>>> X varies (the same with /dev/tapY).. >>>> >>>>> >>>>>> macvtap has its own major number, but tuntap use "misc" (10) major >>>>>> number. >>>> >>>> Another question: it is possible to use the "fd=" parameter with macvtap >>>> as macvtap creates a /dev/tapY device, but how to do that with tuntap >>>> that does not create a /dev/tapY device? >>> >>> >>> I think we should step back and ask why we need to check this at all. >>> >>> IMHO, if the passed-in FD works with the syscalls that tap-linux.c >>> is executing, then that shows the FD is suitable for QEMU. The problem >>> is that many of the tap APIs don't use "Error **errp" parameters to >>> report errors, so we can't catch the failures. IOW, instead of checking >>> the FD major/minor number, we should make the existing code be better >>> at reporting errors, so they can be fed back to the QMP console >>> gracefully. >> >> The problem here is the very first operation of net_init_tap() is a >> qemu_set_nonblock() that has an assert() and crashes QEMU. >> >> It's why I was only checking for the validity of the file descriptor, >> not if it is a tap device or not. > > Yep, checking that it is really a FD is sufficient to avoid the > assert in nonblock. > > As for whether it is really a tap device, I think we just need to > improve error reporting of the functions that come later, instead > of doing a literal "is it a tap" check.
I agree. I will update my patches to have a series with my patch checking for the validity of fd and another patch to return the errors to QMP from the tap functions. > That's what I'd tried in my old patch from a few years back > > https://patchwork.kernel.org/patch/10029443/ > > I can't remember why we didn't merge this back then Jason already gave the link in the thread. I'm going to try to use your patch in my series. Thanks, Laurent
