drm: block userspace under allocating buffer and having drivers overwrite it (v2)

With the current screwed but its ABI, ioctls for the drm, Linus pointed out that we could allow userspace to specify the allocation size, but we pass it to the driver which then uses it blindly to store a struct. Now if userspace specifies the allocation size as smaller than the driver needs, the driver can possibly overwrite memory.

This patch restructures the driver ioctls so we store the structure size we are expecting, and make sure we allocate at least that size. The copy from/to userspace are still restricted to the size the user specifies, this allows ioctl structs to grow on both sides of the equation.

Up until now we didn't really use the DRM_IOCTL defines in the kernel, so this cleans them up and adds them for nouveau.

v2:
fix nouveau pushbuf arg (thanks to Ben for pointing it out)

Reported-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 17 insertions, 6 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 3644c94c0a1..84da748555b 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -55,6 +55,9 @@
 static int drm_version(struct drm_device *dev, void *data,
 		       struct drm_file *file_priv);
 
+#define DRM_IOCTL_DEF(ioctl, _func, _flags) \
+	[DRM_IOCTL_NR(ioctl)] = {.cmd = ioctl, .func = _func, .flags = _flags, .cmd_drv = 0}
+
 /** Ioctl table */
 static struct drm_ioctl_desc drm_ioctls[] = {
 	DRM_IOCTL_DEF(DRM_IOCTL_VERSION, drm_version, 0),
@@ -421,6 +424,7 @@ long drm_ioctl(struct file *filp,
 	int retcode = -EINVAL;
 	char stack_kdata[128];
 	char *kdata = NULL;
+	unsigned int usize, asize;
 
 	dev = file_priv->minor->dev;
 	atomic_inc(&dev->ioctl_count);
@@ -436,11 +440,18 @@ long drm_ioctl(struct file *filp,
 	    ((nr < DRM_COMMAND_BASE) || (nr >= DRM_COMMAND_END)))
 		goto err_i1;
 	if ((nr >= DRM_COMMAND_BASE) && (nr < DRM_COMMAND_END) &&
-	    (nr < DRM_COMMAND_BASE + dev->driver->num_ioctls))
+	    (nr < DRM_COMMAND_BASE + dev->driver->num_ioctls)) {
+		u32 drv_size;
 		ioctl = &dev->driver->ioctls[nr - DRM_COMMAND_BASE];
+		drv_size = _IOC_SIZE(ioctl->cmd_drv);
+		usize = asize = _IOC_SIZE(cmd);
+		if (drv_size > asize)
+			asize = drv_size;
+	}
 	else if ((nr >= DRM_COMMAND_END) || (nr < DRM_COMMAND_BASE)) {
 		ioctl = &drm_ioctls[nr];
 		cmd = ioctl->cmd;
+		usize = asize = _IOC_SIZE(cmd);
 	} else
 		goto err_i1;
 
@@ -460,10 +471,10 @@ long drm_ioctl(struct file *filp,
 		retcode = -EACCES;
 	} else {
 		if (cmd & (IOC_IN | IOC_OUT)) {
-			if (_IOC_SIZE(cmd) <= sizeof(stack_kdata)) {
+			if (asize <= sizeof(stack_kdata)) {
 				kdata = stack_kdata;
 			} else {
-				kdata = kmalloc(_IOC_SIZE(cmd), GFP_KERNEL);
+				kdata = kmalloc(asize, GFP_KERNEL);
 				if (!kdata) {
 					retcode = -ENOMEM;
 					goto err_i1;
@@ -473,12 +484,12 @@ long drm_ioctl(struct file *filp,
 
 		if (cmd & IOC_IN) {
 			if (copy_from_user(kdata, (void __user *)arg,
-					   _IOC_SIZE(cmd)) != 0) {
+					   usize) != 0) {
 				retcode = -EFAULT;
 				goto err_i1;
 			}
 		} else
-			memset(kdata, 0, _IOC_SIZE(cmd));
+			memset(kdata, 0, usize);
 
 		if (ioctl->flags & DRM_UNLOCKED)
 			retcode = func(dev, kdata, file_priv);
@@ -490,7 +501,7 @@ long drm_ioctl(struct file *filp,
 
 		if (cmd & IOC_OUT) {
 			if (copy_to_user((void __user *)arg, kdata,
-					 _IOC_SIZE(cmd)) != 0)
+					 usize) != 0)
 				retcode = -EFAULT;
 		}
 	}