Subject: Re: [PATCH net-next 1/4] sctp: check addr_size with sa_family_t size in __sctp_setsockopt_connectx

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Wed, 24 Jul 2019 16:12:43 -0300

On Wed, Jul 24, 2019 at 04:05:43PM -0300, Marcelo Ricardo Leitner wrote:
> On Wed, Jul 24, 2019 at 02:44:56PM -0400, Neil Horman wrote:
> > On Wed, Jul 24, 2019 at 09:49:07AM -0300, Marcelo Ricardo Leitner wrote:
> > > On Wed, Jul 24, 2019 at 09:36:50AM -0300, Marcelo Ricardo Leitner wrote:
> > > > On Wed, Jul 24, 2019 at 07:22:35AM -0400, Neil Horman wrote:
> > > > > On Wed, Jul 24, 2019 at 03:21:12PM +0800, Xin Long wrote:
> > > > > > On Tue, Jul 23, 2019 at 11:25 PM Neil Horman <[email-masked]> wrote:
> > > > > > >
> > > > > > > On Tue, Jul 23, 2019 at 01:37:57AM +0800, Xin Long wrote:
> > > > > > > > Now __sctp_connect() is called by __sctp_setsockopt_connectx() and
> > > > > > > > sctp_inet_connect(), the latter has done addr_size check with size
> > > > > > > > of sa_family_t.
> > > > > > > >
> > > > > > > > In the next patch to clean up __sctp_connect(), we will remove
> > > > > > > > addr_size check with size of sa_family_t from __sctp_connect()
> > > > > > > > for the 1st address.
> > > > > > > >
> > > > > > > > So before doing that, __sctp_setsockopt_connectx() should do
> > > > > > > > this check first, as sctp_inet_connect() does.
> > > > > > > >
> > > > > > > > Signed-off-by: Xin Long <[email-masked]>
> > > > > > > > ---
> > > > > > > >  net/sctp/socket.c | 2 +-
> > > > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > > >
> > > > > > > > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > > > > > > > index aa80cda..5f92e4a 100644
> > > > > > > > --- a/net/sctp/socket.c
> > > > > > > > +++ b/net/sctp/socket.c
> > > > > > > > @@ -1311,7 +1311,7 @@ static int __sctp_setsockopt_connectx(struct sock *sk,
> > > > > > > >       pr_debug("%s: sk:%p addrs:%p addrs_size:%d\n",
> > > > > > > >                __func__, sk, addrs, addrs_size);
> > > > > > > >
> > > > > > > > -     if (unlikely(addrs_size <= 0))
> > > > > > > > +     if (unlikely(addrs_size < sizeof(sa_family_t)))
> > > > > > > I don't think this is what you want to check for here.  sa_family_t is
> > > > > > > an unsigned short, and addrs_size is the number of bytes in the addrs
> > > > > > > array.  The addrs array should be at least the size of one struct
> > > > > > > sockaddr (16 bytes iirc), and, if larger, should be a multiple of
> > > > > > > sizeof(struct sockaddr)
> > > > > > sizeof(struct sockaddr) is not the right value to check either.
> > > > > > 
> > > > > > The proper check will be done later in __sctp_connect():
> > > > > > 
> > > > > >         af = sctp_get_af_specific(daddr->sa.sa_family);
> > > > > >         if (!af || af->sockaddr_len > addrs_size)
> > > > > >                 return -EINVAL;
> > > > > > 
> > > > > > So the check 'addrs_size < sizeof(sa_family_t)' in this patch is
> > > > > > just to make sure daddr->sa.sa_family is accessible. the same
> > > > > > check is also done in sctp_inet_connect().
> > > > > > 
> > > > > That doesn't make much sense, if the proper check is done in __sctp_connect with
> > > > > the size of the families sockaddr_len, then we don't need this check at all, we
> > > > > can just let memdup_user take the fault on copy_to_user and return -EFAULT.  If
> > > > > we get that from memdup_user, we know its not accessible, and can bail out.
> > > > > 
> > > > > About the only thing we need to check for here is that addr_len isn't some
> > > > > absurdly high value (i.e. a negative value), so that we avoid trying to kmalloc
> > > > > upwards of 2G in memdup_user.  Your change does that just fine, but its no
> > > > > better or worse than checking for <=0
> > > > 
> > > > One can argue that such check against absurdly high values is random
> > > > and not effective, as 2G can be somewhat reasonable on 8GB systems but
> > > > certainly isn't on 512MB ones. On that, kmemdup_user() will also fail
> > > > gracefully as it uses GFP_USER and __GFP_NOWARN.
> > > > 
> > > > The original check is more for protecting for sane usage of the
> > > > variable, which is an int, and a negative value is questionable. We
> > > > could cast, yes, but.. was that really the intent of the application?
> > > > Probably not.
> > > 
> > > Though that said, I'm okay with the new check here: a quick sanity
> > > check that can avoid expensive calls to kmalloc(), while more refined
> > > check is done later on.
> > > 
> > I agree a sanity check makes sense, just to avoid allocating a huge value
> > (even 2G is absurd on many systems), however, I'm not super comfortable with
> > checking for the value being less than 16 (sizeof(sa_family_t)).  The zero check
> 
> 16 bits you mean then, per
> include/uapi/linux/socket.h
> typedef unsigned short __kernel_sa_family_t;
> include/linux/socket.h
> typedef __kernel_sa_family_t    sa_family_t;
> 
> > is fairly obvious given the signed nature of the lengh field, this check makes
> > me wonder what exactly we are checking for.
> 
> A minimum viable buffer without doing more extensive tests. Beyond
> sa_family, we need to parse sa_family and then that's left for later.
> Perhaps a comment helps, something like
> 	/* Check if we have at least the family type in there */
> ?

Hm, then this could be
-     if (unlikely(addrs_size <= 0))
+     if (unlikely(addrs_size < sizeof(struct sockaddr_in)))
(ipv4)
As it can't be smaller than that, always.

> 
>   Marcelo
> 
> > 
> > Neil
> > 
> > > > 
> > > > > 
> > > > > Neil
> > > > > 
> > > > > > >
> > > > > > > Neil
> > > > > > >
> > > > > > > >               return -EINVAL;
> > > > > > > >
> > > > > > > >       kaddrs = memdup_user(addrs, addrs_size);
> > > > > > > > --
> > > > > > > > 2.1.0
> > > > > > > >
> > > > > > > >
> > > > > > 
> > > 
>