mirror of https://gitee.com/openkylin/linux.git
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
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@ -135,6 +135,7 @@ static int drm_set_master(struct drm_device *dev, struct drm_file *fpriv,
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}
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}
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fpriv->was_master = (ret == 0);
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return ret;
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}
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@ -174,12 +175,72 @@ static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
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return ret;
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}
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/*
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* In the olden days the SET/DROP_MASTER ioctls used to return EACCES when
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* CAP_SYS_ADMIN was not set. This was used to prevent rogue applications
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* from becoming master and/or failing to release it.
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*
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* At the same time, the first client (for a given VT) is _always_ master.
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* Thus in order for the ioctls to succeed, one had to _explicitly_ run the
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* application as root or flip the setuid bit.
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*
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* If the CAP_SYS_ADMIN was missing, no other client could become master...
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* EVER :-( Leading to a) the graphics session dying badly or b) a completely
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* locked session.
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*
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*
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* As some point systemd-logind was introduced to orchestrate and delegate
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* master as applicable. It does so by opening the fd and passing it to users
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* while in itself logind a) does the set/drop master per users' request and
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* b) * implicitly drops master on VT switch.
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*
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* Even though logind looks like the future, there are a few issues:
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* - some platforms don't have equivalent (Android, CrOS, some BSDs) so
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* root is required _solely_ for SET/DROP MASTER.
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* - applications may not be updated to use it,
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* - any client which fails to drop master* can DoS the application using
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* logind, to a varying degree.
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*
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* * Either due missing CAP_SYS_ADMIN or simply not calling DROP_MASTER.
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*
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*
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* Here we implement the next best thing:
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* - ensure the logind style of fd passing works unchanged, and
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* - allow a client to drop/set master, iff it is/was master at a given point
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* in time.
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*
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* Note: DROP_MASTER cannot be free for all, as an arbitrator user could:
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* - DoS/crash the arbitrator - details would be implementation specific
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* - open the node, become master implicitly and cause issues
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*
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* As a result this fixes the following when using root-less build w/o logind
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* - startx
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* - weston
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* - various compositors based on wlroots
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*/
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static int
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drm_master_check_perm(struct drm_device *dev, struct drm_file *file_priv)
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{
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if (file_priv->pid == task_pid(current) && file_priv->was_master)
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return 0;
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if (!capable(CAP_SYS_ADMIN))
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return -EACCES;
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return 0;
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}
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int drm_setmaster_ioctl(struct drm_device *dev, void *data,
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struct drm_file *file_priv)
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{
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int ret = 0;
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mutex_lock(&dev->master_mutex);
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ret = drm_master_check_perm(dev, file_priv);
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if (ret)
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goto out_unlock;
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if (drm_is_current_master(file_priv))
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goto out_unlock;
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@ -224,6 +285,12 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
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int ret = -EINVAL;
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mutex_lock(&dev->master_mutex);
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ret = drm_master_check_perm(dev, file_priv);
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if (ret)
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goto out_unlock;
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ret = -EINVAL;
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if (!drm_is_current_master(file_priv))
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goto out_unlock;
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@ -599,8 +599,8 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
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DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_SET_SAREA_CTX, drm_legacy_setsareactx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
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DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_GET_SAREA_CTX, drm_legacy_getsareactx, DRM_AUTH),
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DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, DRM_ROOT_ONLY),
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DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, DRM_ROOT_ONLY),
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DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, 0),
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DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, 0),
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DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_ADD_CTX, drm_legacy_addctx, DRM_AUTH|DRM_ROOT_ONLY),
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DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_RM_CTX, drm_legacy_rmctx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
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@ -201,6 +201,17 @@ struct drm_file {
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*/
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bool writeback_connectors;
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/**
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* @was_master:
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*
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* This client has or had, master capability. Protected by struct
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* &drm_device.master_mutex.
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*
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* This is used to ensure that CAP_SYS_ADMIN is not enforced, if the
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* client is or was master in the past.
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*/
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bool was_master;
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/**
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* @is_master:
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*
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