Re: [PATCH v2 07/20] randstruct: Whitelist big_key path struct overloading

From: Kees Cook <keescook@chromium.org>
To: Christoph Hellwig <hch@infradead.org>,
	David Howells <dhowells@redhat.com>
Cc: "kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	Laura Abbott <labbott@redhat.com>,
	"x86@kernel.org" <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 07/20] randstruct: Whitelist big_key path struct overloading
Date: Sun, 28 May 2017 09:59:31 -0700	[thread overview]
Message-ID: <CAGXu5j+LSrLzZSWLozjGvjEQV6P-Mgsj9Fgdp9wuiFxV=JrYgA@mail.gmail.com> (raw)
In-Reply-To: <20170528081249.GD22193@infradead.org>

On Sun, May 28, 2017 at 1:12 AM, Christoph Hellwig <hch@infradead.org> wrote:
> What about the untested patch below to just fix the issue?
>
> ---
> From e9eb519c854d2f3d16a4def492577a883246e290 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@lst.de>
> Date: Sun, 28 May 2017 11:03:34 +0300
> Subject: security/keys: don't cast union key_payload
>
> Instead store the individual pointers in struct path.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Yeah, this is less invasive than what I'd proposed to David to fix it
earlier. David, does this look okay to you?

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

-Kees

> ---
>  security/keys/big_key.c | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/security/keys/big_key.c b/security/keys/big_key.c
> index 835c1ab30d01..06f2cd07dbd7 100644
> --- a/security/keys/big_key.c
> +++ b/security/keys/big_key.c
> @@ -26,8 +26,8 @@
>   */
>  enum {
>         big_key_data,
> -       big_key_path,
> -       big_key_path_2nd_part,
> +       big_key_path_mnt,
> +       big_key_path_dentry,
>         big_key_len,
>  };
>
> @@ -118,12 +118,16 @@ static int big_key_crypt(enum big_key_op op, u8 *data, size_t datalen, u8 *key)
>         return ret;
>  }
>
> +#define PATH_FROM_PAYLOAD(p) {                         \
> +       .mnt    = (p)->data[big_key_path_mnt],          \
> +       .dentry = (p)->data[big_key_path_dentry],       \
> +}
> +
>  /*
>   * Preparse a big key
>   */
>  int big_key_preparse(struct key_preparsed_payload *prep)
>  {
> -       struct path *path = (struct path *)&prep->payload.data[big_key_path];
>         struct file *file;
>         u8 *enckey;
>         u8 *data = NULL;
> @@ -190,9 +194,10 @@ int big_key_preparse(struct key_preparsed_payload *prep)
>                 /* Pin the mount and dentry to the key so that we can open it again
>                  * later
>                  */
> +               path_get(&file->f_path);
>                 prep->payload.data[big_key_data] = enckey;
> -               *path = file->f_path;
> -               path_get(path);
> +               prep->payload.data[big_key_path_mnt] = file->f_path.mnt;
> +               prep->payload.data[big_key_path_dentry] = file->f_path.dentry;
>                 fput(file);
>                 kfree(data);
>         } else {
> @@ -222,9 +227,9 @@ int big_key_preparse(struct key_preparsed_payload *prep)
>  void big_key_free_preparse(struct key_preparsed_payload *prep)
>  {
>         if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
> -               struct path *path = (struct path *)&prep->payload.data[big_key_path];
> +               struct path path = PATH_FROM_PAYLOAD(&prep->payload);
>
> -               path_put(path);
> +               path_put(&path);
>         }
>         kfree(prep->payload.data[big_key_data]);
>  }
> @@ -235,13 +240,13 @@ void big_key_free_preparse(struct key_preparsed_payload *prep)
>   */
>  void big_key_revoke(struct key *key)
>  {
> -       struct path *path = (struct path *)&key->payload.data[big_key_path];
> +       struct path path = PATH_FROM_PAYLOAD(&key->payload);
>
>         /* clear the quota */
>         key_payload_reserve(key, 0);
>         if (key_is_instantiated(key) &&
>             (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
> -               vfs_truncate(path, 0);
> +               vfs_truncate(&path, 0);
>  }
>
>  /*
> @@ -252,11 +257,11 @@ void big_key_destroy(struct key *key)
>         size_t datalen = (size_t)key->payload.data[big_key_len];
>
>         if (datalen > BIG_KEY_FILE_THRESHOLD) {
> -               struct path *path = (struct path *)&key->payload.data[big_key_path];
> +               struct path path = PATH_FROM_PAYLOAD(&key->payload);
>
> -               path_put(path);
> -               path->mnt = NULL;
> -               path->dentry = NULL;
> +               path_put(&path);
> +               key->payload.data[big_key_path_mnt] = NULL;
> +               key->payload.data[big_key_path_dentry] = NULL;
>         }
>         kfree(key->payload.data[big_key_data]);
>         key->payload.data[big_key_data] = NULL;
> @@ -290,7 +295,7 @@ long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
>                 return datalen;
>
>         if (datalen > BIG_KEY_FILE_THRESHOLD) {
> -               struct path *path = (struct path *)&key->payload.data[big_key_path];
> +               struct path path = PATH_FROM_PAYLOAD(&key->payload);
>                 struct file *file;
>                 u8 *data;
>                 u8 *enckey = (u8 *)key->payload.data[big_key_data];
> @@ -300,7 +305,7 @@ long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
>                 if (!data)
>                         return -ENOMEM;
>
> -               file = dentry_open(path, O_RDONLY, current_cred());
> +               file = dentry_open(&path, O_RDONLY, current_cred());
>                 if (IS_ERR(file)) {
>                         ret = PTR_ERR(file);
>                         goto error;
> --
> 2.11.0
>

-- 
Kees Cook
Pixel Security