On Fri, 18 Oct 2019 17:47:49 +0200 Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > On Fri, Oct 18, 2019 at 4:34 PM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > > > > On Fri, 18 Oct 2019 16:19:33 +0200 > > Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > > > > On Fri, Oct 18, 2019 at 3:43 PM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote: > > > > > > > > On Fri, 18 Oct 2019 07:54:50 -0400 > > > > "Drew DeVault" <sir@xxxxxxxxx> wrote: > > > > > > > > > On Fri Oct 18, 2019 at 12:21 PM Pekka Paalanen wrote: > > > > > > One thing I did not know the last time was that apparently > > > > > > systemd-logind may not like to give out non-master DRM fds. That might > > > > > > need fixing in logind implementations. I hope someone would step up to > > > > > > look into that. > > > > > > > > > > > > This protocol aims to deliver a harmless "read-only" DRM device file > > > > > > description to a client, so that the client can enumerate all DRM > > > > > > resources, fetch EDID and other properties to be able to decide which > > > > > > connector it would want to lease. The client should not be able to > > > > > > change any KMS state through this fd, and it should not be able to e.g. > > > > > > spy on display contents. The assumption is that a non-master DRM fd > > > > > > from a fresh open() would be fine for this, but is it? > > > > > > > > > > What I do for wlroots is call drmGetDeviceNameFromFd2, which returns the > > > > > path to the device file, and then I open() it and use > > > > > drmIsMaster/drmDropMaster to make sure it's not a master fd. This seems > > > > > to work correctly in practice. > > > > > > > > That is nice. > > > > > > > > Personally I'm specifically worried about a setup where the user has no > > > > access permissions to open the DRM device node directly, as is (or > > > > should be) the case with input devices. > > > > > > > > However, since DRM has the master concept which input devices do not, > > > > maybe there is no reason to prevent a normal user from opening a DRM > > > > device directly. That is, if our assumption that a non-master DRM fd is > > > > harmless holds. > > > > > > > > (Wayland display servers usually run as a normal user, while logind > > > > or another service runs with elevated privileges and opens input and > > > > DRM devices on behalf of the display server.) > > > > > > So the rules are (if I'm not making a mistake) > > > - If you're not CAP_SYS_ADMIN you can't get/drop_master. > > > > Hi, > > > > not able to drop, yikes. So if someone pokes the Wayland DRM leasing > > interface while the display server is VT switched away (does not have > > DRM master), and maybe no-one else has DRM master either (you're > > hacking in VT text mode), then a new DRM fd would be master with no way > > out? > > > > So Wayland display servers should make sure they have master themselves > > before sending a supposedly non-master DRM fd to anyone else. I wonder > > if the Wayland protocol extension needs to consider that the compositor > > might not be able to send any fd soon. Being able to defer sending the > > fd should probably be mentioned in the protocol spec, so that clients > > do not expect a simple roundtrip to be enough to ensure the fd has > > arrived. > > > > > - This is a bit awkward, since non-root can become a master, when > > > there's no other master right at this point. So if you want to be able > > > to do this, we should probably clarify this part of the uapi somehow > > > (either de-priv drop_master or make sure non-root can't become master, > > > but the latter probably will break something somewhere). Plus igts to > > > lock down this behaviour. Note that if logind does a vt switch there's > > > a race window where no one is master and you might be able to squeeze > > > in. So perhaps we do want to stop this behaviour and require > > > CAP_SYS_ADMIN to become master, even accidentally. > > > > That would close the loophole that Ville mentioned, too, otherwise > > distributions should aim to not give permissions to open the DRM device > > node. > > I'm kinda wondering whether we have to do this as a security fix, with > maybe a module option to get the old behaviour back for those who > need/want that. But I don't even know whom/where to ping for logind > folks ... I would guess https://lists.freedesktop.org/mailman/listinfo/systemd-devel It's fairly low traffic nowadays. > > > - I thought you can always re-open your own fd through proc? Which > > > should be good enough for this use-case here. > > > > We can? And that creates a new file description the same way as open() > > in the original device node? > > I dreamed, it's just a normal symlink, nothing fancy. D'oh. So we have two options: ensure logind is happy to deliver also non-master DRM fds, or prohibit an open() on a DRM device node from becoming master without CAP_SYS_ADMIN or something, right? Drew does have a point though: if a non-CAP_SYS_ADMIN process gains DRM master, it has no way to drop master, does it? Which means it's impossible to VT-switch away from it and have something else gain master? Still, I can see how someone could rely on gaining master via open() and as non-root. Thanks, pq
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