[PATCH 2/4] fbdev: defio: Keep module reference from VMAs

[Thomas Zimmermann <tzimmermann@suse.de>]

Acquire a module reference on each mmap and VMA open; hold it until
the kernel closes the VMA. Protects against unloading the module
while user space still has a mapping of the graphics memory. The
VMA page-fault handling would then call into undefined code.

This situation can happen if the underlying device has been unplugged
and the driver has been unloaded. It would then be possible to trigger
the bug by unloading the fbdev core module.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/video/fbdev/core/fb_defio.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index 0b099a89a823..331b1a9fe485 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -14,6 +14,7 @@
 #include <linux/export.h>
 #include <linux/string.h>
 #include <linux/mm.h>
+#include <linux/module.h>
 #include <linux/vmalloc.h>
 #include <linux/delay.h>
 #include <linux/interrupt.h>
@@ -83,6 +84,7 @@ static void fb_deferred_io_vm_open(struct vm_area_struct *vma)
 {
 	struct fb_deferred_io_state *fbdefio_state = vma->vm_private_data;
 
+	WARN_ON_ONCE(!try_module_get(THIS_MODULE));
 	fb_deferred_io_state_get(fbdefio_state);
 }
 
@@ -91,6 +93,7 @@ static void fb_deferred_io_vm_close(struct vm_area_struct *vma)
 	struct fb_deferred_io_state *fbdefio_state = vma->vm_private_data;
 
 	fb_deferred_io_state_put(fbdefio_state);
+	module_put(THIS_MODULE);
 }
 
 static struct page *fb_deferred_io_get_page(struct fb_info *info, unsigned long offs)
@@ -335,6 +338,9 @@ int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
 {
 	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
 
+	if (!try_module_get(THIS_MODULE))
+		return -EINVAL;
+
 	vma->vm_ops = &fb_deferred_io_vm_ops;
 	vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP);
 	if (!(info->flags & FBINFO_VIRTFB))
-- 
2.52.0