linux-kernel - [ 006/153] [media] omap_vout: find_vma() needs ->mmap

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Message-Id: <20130304033708.288359852@decadent.org.uk>
Date:	Mon, 04 Mar 2013 03:37:13 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Al Viro <viro@...IV.linux.org.uk>,
	Al Viro <viro@...iv.linux.org.uk>,
	Sakari Ailus <sakari.ailus@....fi>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Archit Taneja <archit@...com>,
	Prabhakar Lad <prabhakar.lad@...com>,
	Mauro Carvalho Chehab <mchehab@...hat.com>
Subject: [ 006/153] [media] omap_vout: find_vma() needs ->mmap_sem held

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@...IV.linux.org.uk>

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@...iv.linux.org.uk>
Cc: Sakari Ailus <sakari.ailus@....fi>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Archit Taneja <archit@...com>
Cc: Prabhakar Lad <prabhakar.lad@...com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@...hat.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- 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(u3
 	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);


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