2019-05-08 08:35:51

by Sahitya Tummala

Subject: [PATCH v2] 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 <[email protected]>
---
v2:
add a comment in dx_release()

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

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 980166a..5d9ffa8 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -871,12 +871,15 @@ 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);
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.

2019-05-11 17:21:53

by Theodore Ts'o

Subject: Re: [PATCH v2] ext4: fix use-after-free in dx_release()

On Wed, May 08, 2019 at 02:04:03PM +0530, Sahitya Tummala wrote:
> 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 <[email protected]>

Nice catch. Thanks, applied.

- Ted