[PATCH] drm/gma500: bound the fbdev fault loop to the framebuffer

[Baul Lee <baul.lee@xbow.com>]

psb_fbdev_vm_fault() maps as many frames as the caller asked for:

	unsigned long pfn = info->fix.smem_start >> PAGE_SHIFT;
	unsigned long page_num = vma_pages(vma);

	for (i = 0; i < page_num; ++i) {
		err = vmf_insert_mixed(vma, address, pfn);
		...
		address += PAGE_SIZE;
		++pfn;
	}

The trip count is the VMA length and the pfn walks up from the
framebuffer base.  info->fix.smem_len, the extent of the framebuffer, is
not read anywhere in the handler, so a mapping longer than the
framebuffer installs present PTEs for the physical frames above it.

Nothing else on the path bounds the length.  psb_fbdev_fb_mmap() only
requires vm_pgoff to be zero, and fb_mmap() passes the length through.

On x86_64 an unprivileged process in group video mapped 4800 pages of a
600-page framebuffer; every page was present and writable, and the 4200
past the object were the frames above it in physical memory.  Nothing is
logged: the kernel installs valid PTEs and every access through them
comes from userspace.

Reject a fault whose page offset is already past the framebuffer and
clamp the loop to the frames the framebuffer owns.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 0867b42113ec ("staging: gma500: Intel GMA500 staging driver")
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
 drivers/gpu/drm/gma500/fbdev.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/gma500/fbdev.c b/drivers/gpu/drm/gma500/fbdev.c
index d1e93588234f..393a939a0c37 100644
--- a/drivers/gpu/drm/gma500/fbdev.c
+++ b/drivers/gpu/drm/gma500/fbdev.c
@@ -26,9 +26,15 @@ static vm_fault_t psb_fbdev_vm_fault(struct vm_fault *vmf)
 	unsigned long address = vmf->address - (vmf->pgoff << PAGE_SHIFT);
 	unsigned long pfn = info->fix.smem_start >> PAGE_SHIFT;
 	vm_fault_t err = VM_FAULT_SIGBUS;
+	unsigned long obj_pages = info->fix.smem_len >> PAGE_SHIFT;
 	unsigned long page_num = vma_pages(vma);
 	unsigned long i;
 
+	if (vmf->pgoff >= obj_pages)
+		return VM_FAULT_SIGBUS;
+	if (page_num > obj_pages)
+		page_num = obj_pages;
+
 	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
 
 	for (i = 0; i < page_num; ++i) {
-- 
2.50.1 (Apple Git-155)