drm: rework SET_MASTER and DROP_MASTER perm handling

This commit reworks the permission handling of the two ioctls. In
particular it enforced the CAP_SYS_ADMIN check only, if:
 - we're issuing the ioctl from process other than the one which opened
the node, and
 - we are, or were master in the past

This ensures that we:
 - do not regress the systemd-logind style of DRM_MASTER arbitrator
 - allow applications which do not use systemd-logind to drop their
master capabilities (and regain them at later point) ... w/o running as
root.

See the comment above drm_master_check_perm() for more details.

v1:
 - Tweak wording, fixup all checks, add igt test

v2:
 - Add a few more comments, grammar nitpicks.

Cc: Adam Jackson <ajax@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Testcase: igt/core_setmaster/master-drop-set-user
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200319172930.230583-1-emil.l.velikov@gmail.com
This commit is contained in:
Emil Velikov 2020-03-19 17:29:29 +00:00 committed by Emil Velikov
parent c7ccc1b783
commit 45bc3d26c9
3 changed files with 80 additions and 2 deletions

View File

@ -135,6 +135,7 @@ static int drm_set_master(struct drm_device *dev, struct drm_file *fpriv,
}
}
fpriv->was_master = (ret == 0);
return ret;
}
@ -174,12 +175,72 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
return ret;
}
/*
* 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.
*
*
* 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:
* - some platforms don't have equivalent (Android, CrOS, some BSDs) so
* root is required _solely_ for SET/DROP MASTER.
* - 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.
*
* Note: DROP_MASTER cannot be free for all, as an arbitrator user could:
* - DoS/crash the arbitrator - details would be implementation specific
* - open the node, become master implicitly and cause issues
*
* As a result this fixes the following when using root-less build w/o logind
* - startx
* - 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;
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;
@ -224,6 +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);
if (ret)
goto out_unlock;
ret = -EINVAL;
if (!drm_is_current_master(file_priv))
goto out_unlock;

View File

@ -599,8 +599,8 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_SET_SAREA_CTX, drm_legacy_setsareactx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_GET_SAREA_CTX, drm_legacy_getsareactx, DRM_AUTH),
DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, DRM_ROOT_ONLY),
DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, DRM_ROOT_ONLY),
DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, 0),
DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, 0),
DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_ADD_CTX, drm_legacy_addctx, DRM_AUTH|DRM_ROOT_ONLY),
DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_RM_CTX, drm_legacy_rmctx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),

View File

@ -201,6 +201,17 @@ struct drm_file {
*/
bool writeback_connectors;
/**
* @was_master:
*
* This client has or had, master capability. Protected by struct
* &drm_device.master_mutex.
*
* This is used to ensure that CAP_SYS_ADMIN is not enforced, if the
* client is or was master in the past.
*/
bool was_master;
/**
* @is_master:
*