Commit 8de779dc authored by Rajat Gupta's avatar Rajat Gupta Committed by Helge Deller
Browse files

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



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: default avatarRajat Gupta <rajgupt@qti.qualcomm.com>
Acked-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Signed-off-by: default avatarHelge Deller <deller@gmx.de>
parent d237f719

drivers/video/fbdev/udlfb.c

+30 −1
Original line number Diff line number Diff line
@@ -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);
@@ -358,6 +378,9 @@ 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;

}
@@ -1176,7 +1199,6 @@ 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)

{
@@ -1188,6 +1210,13 @@ 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

+1 −0
Original line number Diff line number Diff line
@@ -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 */