From 45bc3d26c95a8fc63a7d8668ca9e57ef0883351c Mon Sep 17 00:00:00 2001 From: Emil Velikov Date: Thu, 19 Mar 2020 17:29:29 +0000 Subject: 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 Cc: Daniel Vetter Cc: Pekka Paalanen Testcase: igt/core_setmaster/master-drop-set-user Signed-off-by: Emil Velikov Reviewed-by: Adam Jackson Link: https://patchwork.freedesktop.org/patch/msgid/20200319172930.230583-1-emil.l.velikov@gmail.com --- drivers/gpu/drm/drm_auth.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) (limited to 'drivers/gpu/drm/drm_auth.c') diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index 531b876d0ed8..93c57f08bd93 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; } @@ -174,12 +175,72 @@ out_err: 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; -- cgit From 2bf99b22beff545d360fb0cc3a977add147713b6 Mon Sep 17 00:00:00 2001 From: Emil Velikov Date: Thu, 19 Mar 2020 17:29:30 +0000 Subject: drm: error out with EBUSY when device has existing master As requested by Adam, provide different error message for when the device has an existing master. An audit of the following projects, shows that the errno is used only for printf() purposes. xorg/xserver xorg/drivers/xf86-video-ati xorg/drivers/xf86-video-amdgpu xorg/drivers/xf86-video-intel xorg/drivers/xf86-video-tegra xorg/drivers/xf86-video-freedreno xorg/drivers/xf86-video-nouveau xorg/drivers/xf86-video-vmwgfx qt/kwin/plasma gtk/mutter/gnomeshell efl/enlightment Cc: Adam Jackson Suggested-by: Adam Jackson Signed-off-by: Emil Velikov Reviewed-by: Adam Jackson Link: https://patchwork.freedesktop.org/patch/msgid/20200319172930.230583-2-emil.l.velikov@gmail.com --- drivers/gpu/drm/drm_auth.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'drivers/gpu/drm/drm_auth.c') diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index 93c57f08bd93..800ac39f3213 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -245,7 +245,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data, goto out_unlock; if (dev->master) { - ret = -EINVAL; + ret = -EBUSY; goto out_unlock; } -- cgit