omap_vout: find_vma() needs ->mmap_sem held

commit 55ee64b30a38d688232e5eb2860467dddc493573 upstream.

Walking rbtree while it's modified is a Bad Idea(tm); besides,
the result of find_vma() can be freed just as it's getting returned
to caller.  Fortunately, it's easy to fix - just take ->mmap_sem a bit
earlier (and don't bother with find_vma() at all if virtp >= PAGE_OFFSET -
in that case we don't even look at its result).

While we are at it, what prevents VIDIOC_PREPARE_BUF calling
v4l_prepare_buf() -> (e.g) vb2_ioctl_prepare_buf() -> vb2_prepare_buf() ->
__buf_prepare() -> __qbuf_userptr() -> vb2_vmalloc_get_userptr() -> find_vma(),
AFAICS without having taken ->mmap_sem anywhere in process?  The code flow
is bloody convoluted and depends on a bunch of things done by initialization,
so I certainly might've missed something...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Sakari Ailus <sakari.ailus@iki.fi>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Archit Taneja <archit@ti.com>
Cc: Prabhakar Lad <prabhakar.lad@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/media/video/omap/omap_vout.c b/drivers/media/video/omap/omap_vout.c
index ee0d0b3..d345215 100644 (file)
--- a/drivers/media/video/omap/omap_vout.c
+++ b/drivers/media/video/omap/omap_vout.c
@@ -206,19 +206,21 @@ static u32 omap_vout_uservirt_to_phys(u32 virtp)
        struct vm_area_struct *vma;
        struct mm_struct *mm = current->mm;
 
-       vma = find_vma(mm, virtp);
        /* For kernel direct-mapped memory, take the easy way */
-       if (virtp >= PAGE_OFFSET) {
-               physp = virt_to_phys((void *) virtp);
-       } else if (vma && (vma->vm_flags & VM_IO) && vma->vm_pgoff) {
+       if (virtp >= PAGE_OFFSET)
+               return virt_to_phys((void *) virtp);
+
+       down_read(&current->mm->mmap_sem);
+       vma = find_vma(mm, virtp);
+       if (vma && (vma->vm_flags & VM_IO) && vma->vm_pgoff) {
                /* this will catch, kernel-allocated, mmaped-to-usermode
                   addresses */
                physp = (vma->vm_pgoff << PAGE_SHIFT) + (virtp - vma->vm_start);
+               up_read(&current->mm->mmap_sem);
        } else {
                /* otherwise, use get_user_pages() for general userland pages */
                int res, nr_pages = 1;
                struct page *pages;
-               down_read(&current->mm->mmap_sem);
 
                res = get_user_pages(current, current->mm, virtp, nr_pages, 1,
                                0, &pages, NULL);