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

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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>
This commit is contained in:
Rajat Gupta 2026-05-03 20:51:10 -07:00 committed by Helge Deller
parent d237f719b2
commit 8de779dc40
2 changed files with 31 additions and 1 deletions
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31
drivers/video/fbdev/udlfb.c
View File

@ -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
*/

1
include/video/udlfb.h
View File

@ -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 */