Re: [PATCH net] sctp: fix sockopt size check

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Wed, Jul 29, 2015 at 05:07:31PM -0700, David Miller wrote:
> From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> Date: Tue, 28 Jul 2015 11:16:23 -0300
> 
> > The problem is not on being bigger than what we want, but on being
> > smaller, as it causes read of invalid memory.
> > 
> > Note that the struct changes on commit 7e8616d8e773 didn't affect
> > sctp_setsockopt_events one but that's where this check was flipped.
> > 
> > Fixes: 7e8616d8e773 ("[SCTP]: Update AUTH structures to match
> > declarations in draft-16.")
> > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> 
> This makes things worse.
> 
> The copy_from_user() call is bounded by optlen, so if you allow it to
> be any arbitrary large value the user can write past the end of the
> structure, corrupting kernel memory.

Indeed. I should have changed copy_from_user() to copy the size of the
struct too. But then the issue I thought there was, there isn't and it
just allows partial updates, as it won't read any further than optlen.

> No, the test is correct, or at least necessary, as-is.

Yes. Please drop this. Thanks.

  Marcelo