From: Emil Velikov <emil.velikov@xxxxxxxxxxxxx>
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@xxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Pekka Paalanen <ppaalanen@xxxxxxxxx>
Testcase: igt/core_setmaster/master-drop-set-user
Signed-off-by: Emil Velikov <emil.velikov@xxxxxxxxxxxxx>
Reviewed-by: Adam Jackson <ajax@xxxxxxxxxx>
---
drivers/gpu/drm/drm_auth.c | 67 +++++++++++++++++++++++++++++++++++++
drivers/gpu/drm/drm_ioctl.c | 4 +--
include/drm/drm_file.h | 11 ++++++
3 files changed, 80 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index cc9acd986c68..37cac0a221ff 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -135,6 +135,7 @@ static int drm_set_master(struct drm_device *dev, struct drm_file *fpriv,
}
}
+ fpriv->was_master = (ret == 0);
return ret;
}
@@ -179,12 +180,72 @@ 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;
@@ -229,6 +290,12 @@ 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;
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 9e41972c4bbc..73e31dd4e442 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -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),
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index 19df8028a6c4..c4746c9d3619 100644
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -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:
*
--
2.25.1
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