From: Ross Zwisler <ross.zwisler@linux.intel.com>
Date: Fri, 22 Jan 2016 23:10:31 +0000 (-0800)
Subject: dax: fix NULL pointer dereference in __dax_dbg()
X-Git-Tag: v4.5-rc1~12^2~11
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=d4bbe7068b60e9263f08c54e6c2a0166c0f37317;p=linux.git

dax: fix NULL pointer dereference in __dax_dbg()

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@linux.intel.com>
Reported-by: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

diff --git a/fs/dax.c b/fs/dax.c
index 7af879759064..513bba5f1e02 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;