On Fri, 6 Mar 2020 at 14:00, Pekka Paalanen ppaalanen@gmail.com wrote:
On Wed, 19 Feb 2020 13:27:28 +0000 Emil Velikov emil.l.velikov@gmail.com wrote:
From: Emil Velikov emil.velikov@collabora.com
...
+/*
- In the olden days the SET/DROP_MASTER ioctls used to return EACCES when
- CAP_SYS_ADMIN was not set. This was used to prevent rogue applications
- from becoming master and/or failing to release it.
- At the same time, the first client (for a given VT) is _always_ master.
- Thus in order for the ioctls to succeed, one had to _explicitly_ run the
- application as root or flip the setuid bit.
- If the CAP_SYS_ADMIN was missing, no other client could become master...
- EVER :-( Leading to a) the graphics session dying badly or b) a completely
- locked session.
Hi,
sorry I had to trim this email harshly, but Google did not want to deliver it otherwise.
I agree that being able to drop master without CAP_SYS_ADMIN sounds like a good thing.
- As some point systemd-logind was introduced to orchestrate and delegate
- master as applicable. It does so by opening the fd and passing it to users
- while in itself logind a) does the set/drop master per users' request and
- b) * implicitly drops master on VT switch.
- Even though logind looks like the future, there are a few issues:
- using it is not possible on some platforms
- applications may not be updated to use it,
- any client which fails to drop master* can DoS the application using
- logind, to a varying degree.
- Either due missing CAP_SYS_ADMIN or simply not calling DROP_MASTER.
- Here we implement the next best thing:
- ensure the logind style of fd passing works unchanged, and
- allow a client to drop/set master, iff it is/was master at a given point
- in time.
I understand the drop master part, because it is needed to get rid of apps that accidentally gain DRM master because they were the first one to open the DRM device (on a particular VT?). It could happen e.g. if a Wayland client is inspecting DRM devices to figure what it wants to lease while the user has VT-switched to a text-mode VT, I guess. E.g. starting a Wayland VR compositor from a VT for whatever reason.
The set master without CAP_SYS_ADMIN part I don't understand.
As you point out application can drop master for various reasons. One of which may be to say spawn another program which requires master for _non_ modeset reasons. For example: - amdgpu: create a renderer and use the context/process priority override IOCTL - vmwgfx: stream claim/unref (amongst others).
Another case to consider is classic X or Wayland compositor. With CAP_SYS_ADMIN for DROP_MASTER removed, yet retained in SET_MASTER, the IOCTL will fail. Thus: - weston results in frozen session and session switching (have to force kill weston or sudo loginctl kill-session) - depending on the driver, X will work or crash
To make this clearer I'll include //comment sections in the code.
// comment To ensure the application can reclaim its master status, the tweaked CAP_SYS_ADMIN handling is needed for both IOCTLs. Otherwise X or Wayland compositors may freeze or crash as SET_MASTER fails. // comment
- As a result this fixes, the following when using root-less build w/o logind
Why is non-root without any logind-equivalent a use case that should work?
// comment Some platforms don't have equivalent (Android, CrOS, some BSDs), yet root is required _solely_ for DROP/SET MASTER. So tweaking the requirement sounds perfectly reasonable. // comment
Why did DRM set/drop master use to require CAP_SYS_ADMIN in the first place?
I imagine something else could have been introduced instead. Although I cannot find any details or discussion on the topic.
What else happens if we allow DRM set master more than just for CAP_SYS_ADMIN?
If we're talking about removing CAP_SYS_ADMIN all together: - screen scraping by any random application - dead trivial way to DoS your compositor
Does this interact with DRM leasing?
Looks like drmIsMaster() should be unaffected at least: https://patchwork.kernel.org/patch/10776525/
Correct, both are unaffected. All the leasing IOCTLs happen by the active true (aka non-lease) master.
- startx - some drivers work fine regardless
- weston
- various compositors based on wlroots
- */
+static int +drm_master_check_perm(struct drm_device *dev, struct drm_file *file_priv) +{
if (file_priv->pid == task_pid(current) && file_priv->was_master)return 0;In case a helper e.g. logind opens the device, is file_priv->pid then referring to logind regardless of what happens afterwards?
Correct.
if (!capable(CAP_SYS_ADMIN))return -EACCES;return 0;+}
int drm_setmaster_ioctl(struct drm_device *dev, void *data, struct drm_file *file_priv) { int ret = 0;
mutex_lock(&dev->master_mutex);
ret = drm_master_check_perm(dev, file_priv);if (ret)goto out_unlock;if (drm_is_current_master(file_priv)) goto out_unlock;@@ -229,6 +285,12 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data, int ret = -EINVAL;
mutex_lock(&dev->master_mutex);
ret = drm_master_check_perm(dev, file_priv);Why does drop-master need any kind of permission check? Why could it not be free for all?
Consider the arbitrator usecase - be that logind, Xorg (in ancient times) or otherwise.
// comment DROP_MASTER cannot be free for all, as any (say logind) user can: - can DoS/crash the arbitrator - open the node, become master implicitly and cause issues // comment
I've added an IGT subtest to ensure this does not happen.
Let me know if I should include anything more to the commit, than the above comment sections.
Thanks
-Emil