[PATCH 66/72] mtdchar: fix offset overflow detection

[Luis Henriques]

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

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

From: Linus Torvalds <torvalds at linux-foundation.org>

commit 9c603e53d380459fb62fec7cd085acb0b74ac18f upstream.

Sasha Levin has been running trinity in a KVM tools guest, and was able
to trigger the BUG_ON() at arch/x86/mm/pat.c:279 (verifying the range of
the memory type).  The call trace showed that it was mtdchar_mmap() that
created an invalid remap_pfn_range().

The problem is that mtdchar_mmap() does various really odd and subtle
things with the vma page offset etc, and uses the wrong types (and the
wrong overflow) detection for it.

For example, the page offset may well be 32-bit on a 32-bit
architecture, but after shifting it up by PAGE_SHIFT, we need to use a
potentially 64-bit resource_size_t to correctly hold the full value.

Also, we need to check that the vma length plus offset doesn't overflow
before we check that it is smaller than the length of the mtdmap region.

This fixes things up and tries to make the code a bit easier to read.

Reported-and-tested-by: Sasha Levin <levinsasha928 at gmail.com>
Acked-by: Suresh Siddha <suresh.b.siddha at intel.com>
Acked-by: Artem Bityutskiy <dedekind1 at gmail.com>
Cc: David Woodhouse <dwmw2 at infradead.org>
Cc: linux-mtd at lists.infradead.org
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/mtd/mtdchar.c | 48 ++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
index f2f482b..a6e7451 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -1123,6 +1123,33 @@ static unsigned long mtdchar_get_unmapped_area(struct file *file,
 }
 #endif
 
+static inline unsigned long get_vm_size(struct vm_area_struct *vma)
+{
+	return vma->vm_end - vma->vm_start;
+}
+
+static inline resource_size_t get_vm_offset(struct vm_area_struct *vma)
+{
+	return (resource_size_t) vma->vm_pgoff << PAGE_SHIFT;
+}
+
+/*
+ * Set a new vm offset.
+ *
+ * Verify that the incoming offset really works as a page offset,
+ * and that the offset and size fit in a resource_size_t.
+ */
+static inline int set_vm_offset(struct vm_area_struct *vma, resource_size_t off)
+{
+	pgoff_t pgoff = off >> PAGE_SHIFT;
+	if (off != (resource_size_t) pgoff << PAGE_SHIFT)
+		return -EINVAL;
+	if (off + get_vm_size(vma) - 1 < off)
+		return -EINVAL;
+	vma->vm_pgoff = pgoff;
+	return 0;
+}
+
 /*
  * set up a mapping for shared memory segments
  */
@@ -1132,20 +1159,29 @@ static int mtdchar_mmap(struct file *file, struct vm_area_struct *vma)
 	struct mtd_file_info *mfi = file->private_data;
 	struct mtd_info *mtd = mfi->mtd;
 	struct map_info *map = mtd->priv;
-	unsigned long start;
-	unsigned long off;
-	u32 len;
+	resource_size_t start, off;
+	unsigned long len, vma_len;
 
 	if (mtd->type == MTD_RAM || mtd->type == MTD_ROM) {
-		off = vma->vm_pgoff << PAGE_SHIFT;
+		off = get_vm_offset(vma);
 		start = map->phys;
 		len = PAGE_ALIGN((start & ~PAGE_MASK) + map->size);
 		start &= PAGE_MASK;
-		if ((vma->vm_end - vma->vm_start + off) > len)
+		vma_len = get_vm_size(vma);
+
+		/* Overflow in off+len? */
+		if (vma_len + off < off)
+			return -EINVAL;
+		/* Does it fit in the mapping? */
+		if (vma_len + off > len)
 			return -EINVAL;
 
 		off += start;
-		vma->vm_pgoff = off >> PAGE_SHIFT;
+		/* Did that overflow? */
+		if (off < start)
+			return -EINVAL;
+		if (set_vm_offset(vma, off) < 0)
+			return -EINVAL;
 		vma->vm_flags |= VM_IO | VM_RESERVED;
 
 #ifdef pgprot_noncached
-- 
1.8.1.2