Re: [PATCH] fs: Allow opening only regular files during execve().

[Kees Cook]

On Mon, Jan 21, 2019 at 11:15 PM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Dec 12, 2017 at 2:06 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
> > I'm not sure what the fix will be.  Maybe the proc handlers should take a
> > different lock instead of cred_guard_mutex.  Or perhaps execve should check that
> > the file is a regular file before it attempts to open it.
>
> We can easily distinguish open() from execve() and open() from others. ;-)
>
> From a8c559566f743eeec31c3cf5bab7a90b1ff7f78b Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Mon, 21 Jan 2019 13:55:11 +0900
> Subject: [PATCH] fs: Allow opening only regular files during execve().
>
> syzbot is hitting lockdep warning [1] due to trying to open a fifo during
> an execve() operation. But we don't need to open non regular files during
> an execve() operation, for all files which we will need are the executable
> file itself, the interpreter program, and libraries like ld-linux.so.2
> required by the executable file or the interpreter program.
>
> Since the manpage for execve(2) says that execve() returns EACCES when
> the file or a script interpreter is not a regular file, and we set
> current->in_execve flag during an execve() operation, let's bail out
> when current thread tried to open a non regular file during an execve()
> operation.
>
> [1] https://syzkaller.appspot.com/bug?id=b5095bfec44ec84213bac54742a82483aad578ce
>
> Reported-by: syzbot <syzbot+e93a80c1bb7c5c56e522461c149f8bf55eab1b2b@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

I like it!

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  fs/open.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fs/open.c b/fs/open.c
> index 0285ce7dbd51..b2e7b04ade46 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -733,6 +733,12 @@ static int do_dentry_open(struct file *f,
>                 return 0;
>         }
>
> +       /* The file or a script interpreter has to be a regular file. */
> +       if (unlikely(current->in_execve && !S_ISREG(inode->i_mode))) {
> +               error = -EACCES;
> +               goto cleanup_file;
> +       }
> +
>         if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
>                 error = get_write_access(inode);
>                 if (unlikely(error))
> --
> 2.17.1



-- 
Kees Cook