[PATCH v8 1/9] dax: fix NULL pointer dereference in __dax_dbg()

From: Ross Zwisler
Date: Fri Jan 08 2016 - 00:31:57 EST

In __dax_pmd_fault() we currently assume that get_block() will always set
bh.b_bdev and we unconditionally dereference it in __dax_dbg(). This
assumption isn't always true - when called for reads of holes
ext4_dax_mmap_get_block() returns a buffer head where bh->b_bdev is never
set. I hit this BUG while testing the DAX PMD fault path.

Instead, initialize bh.b_bdev before passing bh into get_block(). It is
possible that the filesystem's get_block() will update bh.b_bdev, and this
is fine - we just want to initialize bh.b_bdev to something reasonable so
that the calls to __dax_dbg() work and print something useful.

Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
fs/dax.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/dax.c b/fs/dax.c
index 7af8797..513bba5 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -624,6 +624,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,

memset(&bh, 0, sizeof(bh));
+ bh.b_bdev = inode->i_sb->s_bdev;
block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);

bh.b_size = PMD_SIZE;