[PATCH] proc: Check /proc/$pid/attr/ writes against file opener

[Jann Horn]

On Tue, May 25, 2021 at 9:37 PM Kees Cook <keescook at chromium.org> wrote:
> Fix another "confused deputy" weakness[1]. Writes to /proc/$pid/attr/
> files need to check the opener credentials, since these fds do not
> transition state across execve(). Without this, it is possible to
> trick another process (which may have different credentials) to write
> to its own /proc/$pid/attr/ files, leading to unexpected and possibly
> exploitable behaviors.
>
> [1] https://www.kernel.org/doc/html/latest/security/credentials.html?highlight=confused#open-file-credentials
>
> Fixes: 1da177e4c3f41 ("Linux-2.6.12-rc2")
> Cc: stable at vger.kernel.org
> Signed-off-by: Kees Cook <keescook at chromium.org>
> ---
>  fs/proc/base.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 3851bfcdba56..58bbf334265b 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2703,6 +2703,10 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
>         void *page;
>         int rv;
>
> +       /* A task may only write when it was the opener. */
> +       if (file->f_cred != current_real_cred())
> +               return -EPERM;

With this, if a task forks, the child will then still be able to open
its parent's /proc/$pid/attr/current and trick the parent into writing
to that, right? Is that acceptable? If not, the ->open handler should
probably also check for "current->thread_pid == proc_pid(inode)", or
something like that?