fbdev: udlfb: add vm_ops to dlfb_ops_mmap to prevent use-after-free

commit 8de779d upstream.

dlfb_ops_mmap() uses remap_pfn_range() to map vmalloc framebuffer pages
to userspace but sets no vm_ops on the VMA. This means the kernel cannot
track active mmaps. When dlfb_realloc_framebuffer() replaces the backing
buffer via FBIOPUT_VSCREENINFO, existing mmap PTEs are not invalidated.
On USB disconnect, dlfb_ops_destroy() calls vfree() on the old pages
while userspace PTEs still reference them, resulting in a use-after-free:
the process retains read/write access to freed kernel pages.

Add vm_operations_struct with open/close callbacks that maintain an
atomic mmap_count on struct dlfb_data. In dlfb_realloc_framebuffer(),
check mmap_count and return -EBUSY if the buffer is currently mapped,
preventing buffer replacement while userspace holds stale PTEs.

Tested with PoC using dummy_hcd + raw_gadget USB device emulation.

Signed-off-by: Rajat Gupta <rajgupt@qti.qualcomm.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Rajat Gupta

drivers/video/fbdev/udlfb.c

@@ -321,12 +321,32 @@ static int dlfb_set_video_mode(struct dlfb_data *dlfb,
 	return retval;
 }
 
+static void dlfb_vm_open(struct vm_area_struct *vma)
+{
+	struct dlfb_data *dlfb = vma->vm_private_data;
+
+	atomic_inc(&dlfb->mmap_count);
+}
+
+static void dlfb_vm_close(struct vm_area_struct *vma)
+{
+	struct dlfb_data *dlfb = vma->vm_private_data;
+
+	atomic_dec(&dlfb->mmap_count);
+}
+
+static const struct vm_operations_struct dlfb_vm_ops = {
+	.open  = dlfb_vm_open,
+	.close = dlfb_vm_close,
+};
+
 static int dlfb_ops_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	unsigned long start = vma->vm_start;
 	unsigned long size = vma->vm_end - vma->vm_start;
 	unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
 	unsigned long page, pos;
+	struct dlfb_data *dlfb = info->par;
 
 	if (info->fbdefio)
 		return fb_deferred_io_mmap(info, vma);
@@ -356,6 +376,9 @@ static int dlfb_ops_mmap(struct fb_info *info, struct vm_area_struct *vma)
 			size = 0;
 	}
 
+	vma->vm_ops = &dlfb_vm_ops;
+	vma->vm_private_data = dlfb;
+	atomic_inc(&dlfb->mmap_count);
 	return 0;
 }
 
@@ -1219,7 +1242,6 @@ static void dlfb_deferred_vfree(struct dlfb_data *dlfb, void *mem)
 
 /*
  * Assumes &info->lock held by caller
- * Assumes no active clients have framebuffer open
  */
 static int dlfb_realloc_framebuffer(struct dlfb_data *dlfb, struct fb_info *info, u32 new_len)
 {
@@ -1231,6 +1253,13 @@ static int dlfb_realloc_framebuffer(struct dlfb_data *dlfb, struct fb_info *info
 	new_len = PAGE_ALIGN(new_len);
 
 	if (new_len > old_len) {
+		if (atomic_read(&dlfb->mmap_count) > 0) {
+			dev_warn(info->dev,
+				"refusing realloc: %d active mmaps\n",
+				atomic_read(&dlfb->mmap_count));
+			return -EBUSY;
+		}
+
 		/*
 		 * Alloc system memory for virtual framebuffer
 		 */

include/video/udlfb.h

@@ -56,6 +56,7 @@ struct dlfb_data {
 	spinlock_t damage_lock;
 	struct work_struct damage_work;
 	struct fb_ops ops;
+	atomic_t mmap_count;
 	/* blit-only rendering path metrics, exposed through sysfs */
 	atomic_t bytes_rendered; /* raw pixel-bytes driver asked to render */
 	atomic_t bytes_identical; /* saved effort with backbuffer comparison */