[PATCH] ext4: fix use-after-free in dx_release()

From: Sahitya Tummala
Date: Wed May 08 2019 - 02:14:43 EST

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:

Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
fs/ext4/namei.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 4181c9c..7e6c298 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -871,12 +871,14 @@ static void dx_release(struct dx_frame *frames)
struct dx_root_info *info;
int i;
+ unsigned int indirect_levels;

if (frames[0].bh == NULL)

info = &((struct dx_root *)frames[0].bh->b_data)->info;
- for (i = 0; i <= info->indirect_levels; i++) {
+ indirect_levels = info->indirect_levels;
+ for (i = 0; i <= indirect_levels; i++) {
if (frames[i].bh == NULL)
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