FAILED: patch "[PATCH] ext4: fix use-after-free in dx_release()" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 08fc98a4d6424af66eb3ac4e2cedd2fc927ed436 Mon Sep 17 00:00:00 2001
From: Sahitya Tummala <stummala@codeaurora.org>
Date: Fri, 10 May 2019 22:00:33 -0400
Subject: [PATCH] ext4: fix use-after-free in dx_release()

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 <stummala@codeaurora.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index e917830eae84..ac7457fef9e6 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -872,12 +872,15 @@ static void dx_release(struct dx_frame *frames)
 {
 	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);