2019-07-25 16:04:13

by Jia-Ju Bai

Subject: [PATCH] fs: overlayfs: Fix a possible null-pointer dereference in ovl_free_fs()

In ovl_fill_super(), there is an if statement on line 1607 to check
whether ofs->upper_mnt is NULL:
if (!ofs->upper_mnt)

When ofs->upper_mnt is NULL and d_make_root() on line 1654 fails,
ovl_free_fs() on line 1683 is executed.
In ovl_free_fs(), ofs->upper_mnt is used on line 224:
ovl_inuse_unlock(ofs->upper_mnt->mnt_root);

Thus, a possible null-pointer dereference may occur.

To fix this bug, ofs->upper_mnt is checked before being used in
ovl_free_fs().

This bug is found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <[email protected]>
---
fs/overlayfs/super.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index b368e2e102fa..1d7c3d280834 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -220,7 +220,7 @@ static void ovl_free_fs(struct ovl_fs *ofs)
if (ofs->workdir_locked)
ovl_inuse_unlock(ofs->workbasedir);
dput(ofs->workbasedir);
- if (ofs->upperdir_locked)
+ if (ofs->upperdir_locked && ofs->upper_mnt)
ovl_inuse_unlock(ofs->upper_mnt->mnt_root);
mntput(ofs->upper_mnt);
for (i = 0; i < ofs->numlower; i++) {
--
2.17.0

2019-07-25 18:28:43

by Amir Goldstein

Subject: Re: [PATCH] fs: overlayfs: Fix a possible null-pointer dereference in ovl_free_fs()

On Thu, Jul 25, 2019 at 3:48 PM Jia-Ju Bai <[email protected]> wrote:
>
> In ovl_fill_super(), there is an if statement on line 1607 to check
> whether ofs->upper_mnt is NULL:
> if (!ofs->upper_mnt)
>
> When ofs->upper_mnt is NULL and d_make_root() on line 1654 fails,
> ovl_free_fs() on line 1683 is executed.
> In ovl_free_fs(), ofs->upper_mnt is used on line 224:
> ovl_inuse_unlock(ofs->upper_mnt->mnt_root);
>
> Thus, a possible null-pointer dereference may occur.
>
> To fix this bug, ofs->upper_mnt is checked before being used in
> ovl_free_fs().
>
> This bug is found by a static analysis tool STCheck written by us.
>
> Signed-off-by: Jia-Ju Bai <[email protected]>
> ---
> fs/overlayfs/super.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index b368e2e102fa..1d7c3d280834 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -220,7 +220,7 @@ static void ovl_free_fs(struct ovl_fs *ofs)
> if (ofs->workdir_locked)
> ovl_inuse_unlock(ofs->workbasedir);
> dput(ofs->workbasedir);
> - if (ofs->upperdir_locked)
> + if (ofs->upperdir_locked && ofs->upper_mnt)
> ovl_inuse_unlock(ofs->upper_mnt->mnt_root);
> mntput(ofs->upper_mnt);
> for (i = 0; i < ofs->numlower; i++) {
> --

Can you teach STCheck to know that if upperdir_locked is only set this way:
ofs->upper_mnt = upper_mnt;

err = -EBUSY;
if (ovl_inuse_trylock(ofs->upper_mnt->mnt_root)) {
ofs->upperdir_locked = true;

Then upperdir_locked implies ofs->upper_mnt != NULL?

Whether or not this patch should be applied is not my call,
but the title "possible null-pointer dereference" is certainly not true.

Thanks,
Amir.