Re: [PATCH v2] fs: Allow opening only regular files during execve().

[Andrew Morton]

On Tue, 22 Jan 2019 09:50:48 +0900 Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:

> Al Viro wrote:
> > On Mon, Jan 21, 2019 at 07:14:39PM +0900, Tetsuo Handa 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. ;-)
> > 
> > > +	/* 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;
> > > +	}
> > 
> > We are *NOT* going to use current->in_execve to propagate that information.
> 
> You mean check FMODE_EXEC instead of current->in_execve ?
> 
> >From f80b32e1c3fbd65672387ee441a32ab58db456f8 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Tue, 22 Jan 2019 09:34:35 +0900
> Subject: [PATCH v2] 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 and the interpreter programs like /bin/sh and ld-linux.so.2 .
> 
> Since the manpage for execve(2) says that execve() returns EACCES when
> the file or a script interpreter is not a regular file, and the manpage
> for uselib(2) says that uselib() can return EACCES, and we use FMODE_EXEC
> when opening for execve()/uselib(), let's bail out if a non regular file
> is requested with FMODE_EXEC set.
> 
> [1] https://syzkaller.appspot.com/bug?id=b5095bfec44ec84213bac54742a82483aad578ce
> 
> ...
>
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -733,6 +733,12 @@ static int do_dentry_open(struct file *f,
>  		return 0;
>  	}
>  
> +	/* Any file opened for execve()/uselib() has to be a regular file. */
> +	if (unlikely(f->f_flags & FMODE_EXEC && !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))

This change sounds legitimate for various other reasons, but it's a
concern that this locking error occurred in the first place.  There's a
problem somewhere (probably the pipe code) which may bite us in other
situations, even with this workaround in place.