V4L/DVB (6268): V4L: Fix a lock inversion in generic videobuf code

videobuf_qbuf takes q->lock, and then calls
q->ops->buf_prepare which by design in all drivers calls
videobuf_iolock which calls videobuf_dma_init_user and this
takes current->mm->mmap_sem

on the other hand if user calls mumap from other thread, sys_munmap
takes current->mm->mmap_sem and videobuf_vm_close takes q->lock

Since this can occur only for V4L2_MEMORY_MMAP buffers, take
current->mm->mmap_sem in qbuf, before q->lock, and don't take
current->mm->mmap_sem videobuf_dma_init_user for those buffers

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
http://thread.gmane.org/gmane.comp.video.video4linux/34978/focus=34981
Reviewed-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

1 files changed, 27 insertions, 5 deletions

diff --git a/drivers/media/video/videobuf-dma-sg.c b/drivers/media/video/videobuf-dma-sg.c
index 0939ede831ab..05dd38343fa3 100644
--- a/drivers/media/video/videobuf-dma-sg.c
+++ b/drivers/media/video/videobuf-dma-sg.c
@@ -134,8 +134,8 @@ void videobuf_dma_init(struct videobuf_dmabuf *dma)
         dma->magic = MAGIC_DMABUF;
 }
 
-int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
-                           unsigned long data, unsigned long size)
+static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
+                        int direction, unsigned long data, unsigned long size)
 {
         unsigned long first,last;
         int err, rw = 0;
@@ -160,12 +160,12 @@ int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
 
         dma->varea = (void *) data;
 
-        down_read(&current->mm->mmap_sem);
+
         err = get_user_pages(current,current->mm,
                              data & PAGE_MASK, dma->nr_pages,
                              rw == READ, 1, /* force */
                              dma->pages, NULL);
-        up_read(&current->mm->mmap_sem);
+
         if (err != dma->nr_pages) {
                 dma->nr_pages = (err >= 0) ? err : 0;
                 dprintk(1,"get_user_pages: err=%d [%d]\n",err,dma->nr_pages);
@@ -174,6 +174,17 @@ int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
         return 0;
 }
 
+int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
+                           unsigned long data, unsigned long size)
+{
+        int ret;
+        down_read(&current->mm->mmap_sem);
+        ret = videobuf_dma_init_user_locked(dma, direction, data, size);
+        up_read(&current->mm->mmap_sem);
+
+        return ret;
+}
+
 int videobuf_dma_init_kernel(struct videobuf_dmabuf *dma, int direction,
                              int nr_pages)
 {
@@ -469,13 +480,24 @@ static int __videobuf_iolock (struct videobuf_queue* q,
                                                         pages );
                         if (0 != err)
                                 return err;
-                } else {
+                } else if (vb->memory == V4L2_MEMORY_USERPTR) {
                         /* dma directly to userspace */
                         err = videobuf_dma_init_user( &mem->dma,
                                                       PCI_DMA_FROMDEVICE,
                                                       vb->baddr,vb->bsize );
                         if (0 != err)
                                 return err;
+                } else {
+                        /* NOTE: HACK: videobuf_iolock on V4L2_MEMORY_MMAP
+                        buffers can only be called from videobuf_qbuf
+                        we take current->mm->mmap_sem there, to prevent
+                        locking inversion, so don't take it here */
+
+                        err = videobuf_dma_init_user_locked(&mem->dma,
+                                                      PCI_DMA_FROMDEVICE,
+                                                      vb->baddr, vb->bsize);
+                        if (0 != err)
+                                return err;
                 }
                 break;
         case V4L2_MEMORY_OVERLAY: