[PATCH 5.1 109/128] ext4: fix use-after-free in dx_release()

[Greg Kroah-Hartman]

From: Sahitya Tummala <stumm...@codeaurora.org>

commit 08fc98a4d6424af66eb3ac4e2cedd2fc927ed436 upstream.

The buffer_head (frames[0].bh) and it's corresping page can be
potentially free'd once brelse() is done inside the for loop
but before the for loop exits in dx_release(). It can be free'd
in another context, when the page cache is flushed via
drop_caches_sysctl_handler(). This results into below data abort
when accessing info->indirect_levels in dx_release().

Unable to handle kernel paging request at virtual address ffffffc17ac3e01e
Call trace:
 dx_release+0x70/0x90
 ext4_htree_fill_tree+0x2d4/0x300
 ext4_readdir+0x244/0x6f8
 iterate_dir+0xbc/0x160
 SyS_getdents64+0x94/0x174

Signed-off-by: Sahitya Tummala <stumm...@codeaurora.org>
Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Reviewed-by: Andreas Dilger <adil...@dilger.ca>
Cc: sta...@kernel.org
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>

---
 fs/ext4/namei.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -871,12 +871,15 @@ static void dx_release(struct dx_frame *
 {
        struct dx_root_info *info;
        int i;
+       unsigned int indirect_levels;
 
        if (frames[0].bh == NULL)
                return;
 
        info = &((struct dx_root *)frames[0].bh->b_data)->info;
-       for (i = 0; i <= info->indirect_levels; i++) {
+       /* save local copy, "info" may be freed after brelse() */
+       indirect_levels = info->indirect_levels;
+       for (i = 0; i <= indirect_levels; i++) {
                if (frames[i].bh == NULL)
                        break;
                brelse(frames[i].bh);