[PATCH] ovl: Fix info leak in ovl_lookup_temp()

[PATCH] ovl: Fix info leak in ovl_lookup_temp()

[Richard Weinberger]

The function uses the memory address of a struct dentry as unique id.
While the address-based directory entry is only visible to root
it is IMHO still worth fixing since the temporary name does not have
to be a kernel address. It can be any unique number. Replace it by an
atomic integer which is allowed to wrap around.

Signed-off-by: Richard Weinberger <richard@nod.at>
---
 fs/overlayfs/dir.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 1560fdc..6e00a08 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -14,6 +14,7 @@
 #include <linux/cred.h>
 #include <linux/posix_acl.h>
 #include <linux/posix_acl_xattr.h>
+#include <linux/atomic.h>
 #include "overlayfs.h"
 
 void ovl_cleanup(struct inode *wdir, struct dentry *wdentry)
@@ -37,8 +38,9 @@ struct dentry *ovl_lookup_temp(struct dentry *workdir, struct dentry *dentry)
 {
 	struct dentry *temp;
 	char name[20];
+	static atomic_t temp_id = ATOMIC_INIT(0);
 
-	snprintf(name, sizeof(name), "#%lx", (unsigned long) dentry);
+	snprintf(name, sizeof(name), "#%x", atomic_inc_return(&temp_id));
 
 	temp = lookup_one_len(name, workdir, strlen(name));
 	if (!IS_ERR(temp) && temp->d_inode) {
-- 
2.7.3

Re: [PATCH] ovl: Fix info leak in ovl_lookup_temp()

[Kees Cook]

On Fri, Sep 16, 2016 at 2:45 AM, Richard Weinberger <richard@nod.at> wrote:
> The function uses the memory address of a struct dentry as unique id.
> While the address-based directory entry is only visible to root
> it is IMHO still worth fixing since the temporary name does not have
> to be a kernel address. It can be any unique number. Replace it by an
> atomic integer which is allowed to wrap around.
>
> Signed-off-by: Richard Weinberger <richard@nod.at>

Nice catch!

Reviewed-by: Kees Cook <keescook@chromium.org>

We'll have to remember this for when atomic_t wrap checking gets added
to mark this as "intended to wrap".

Thanks,

-Kees

> ---
>  fs/overlayfs/dir.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 1560fdc..6e00a08 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -14,6 +14,7 @@
>  #include <linux/cred.h>
>  #include <linux/posix_acl.h>
>  #include <linux/posix_acl_xattr.h>
> +#include <linux/atomic.h>
>  #include "overlayfs.h"
>
>  void ovl_cleanup(struct inode *wdir, struct dentry *wdentry)
> @@ -37,8 +38,9 @@ struct dentry *ovl_lookup_temp(struct dentry *workdir, struct dentry *dentry)
>  {
>         struct dentry *temp;
>         char name[20];
> +       static atomic_t temp_id = ATOMIC_INIT(0);
>
> -       snprintf(name, sizeof(name), "#%lx", (unsigned long) dentry);
> +       snprintf(name, sizeof(name), "#%x", atomic_inc_return(&temp_id));
>
>         temp = lookup_one_len(name, workdir, strlen(name));
>         if (!IS_ERR(temp) && temp->d_inode) {
> --
> 2.7.3
>



-- 
Kees Cook
Nexus Security

Re: [PATCH] ovl: Fix info leak in ovl_lookup_temp()

[Miklos Szeredi]

On Fri, Sep 16, 2016 at 10:36 PM, Kees Cook <keescook@chromium.org> wrote:
> On Fri, Sep 16, 2016 at 2:45 AM, Richard Weinberger <richard@nod.at> wrote:
>> The function uses the memory address of a struct dentry as unique id.
>> While the address-based directory entry is only visible to root
>> it is IMHO still worth fixing since the temporary name does not have
>> to be a kernel address. It can be any unique number. Replace it by an
>> atomic integer which is allowed to wrap around.
>>
>> Signed-off-by: Richard Weinberger <richard@nod.at>

Thanks, applied and pushed to #overlayfs-next.

Thanks,
Miklos