From 8de779dc40d35d39fa07387b6f921eb11df0f511 Mon Sep 17 00:00:00 2001 From: Rajat Gupta Date: Sun, 3 May 2026 20:51:10 -0700 Subject: [PATCH] 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: Rajat Gupta Acked-by: Greg Kroah-Hartman Cc: stable@vger.kernel.org Signed-off-by: Helge Deller --- drivers/video/fbdev/udlfb.c | 31 ++++++++++++++++++++++++++++++- include/video/udlfb.h | 1 + 2 files changed, 31 insertions(+), 1 deletion(-) diff --git a/drivers/video/fbdev/udlfb.c b/drivers/video/fbdev/udlfb.c index c341d76bc564..fdbb8671a810 100644 --- a/drivers/video/fbdev/udlfb.c +++ b/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); @@ -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 */ diff --git a/include/video/udlfb.h b/include/video/udlfb.h index 58fb5732831a..ab34790d57ec 100644 --- a/include/video/udlfb.h +++ b/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 */