netdev - Re: [PATCH] net/sctp: Validate parameter size for SCTP_GET_ASSOC

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Date:	Wed, 27 Feb 2013 12:22:55 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Neil Horman <nhorman@...driver.com>
Cc:	netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
	Vlad Yasevich <vyasevich@...il.com>,
	Sridhar Samudrala <sri@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] net/sctp: Validate parameter size for
 SCTP_GET_ASSOC_STATS control message

On Wed, Feb 27, 2013 at 03:09:31PM -0500, Neil Horman wrote:
> On Wed, Feb 27, 2013 at 11:43:51AM -0800, Guenter Roeck wrote:
> > Building sctp may fail with:
> > 
> > In function ‘copy_from_user’,
> >     inlined from ‘sctp_getsockopt_assoc_stats’ at
> >     net/sctp/socket.c:5656:20:
> > arch/x86/include/asm/uaccess_32.h:211:26: error: call to
> >     ‘copy_from_user_overflow’ declared with attribute error: copy_from_user()
> >     buffer size is not provably correct
> > 
> > if built with W=1 due to a missing parameter size validation.
> > 
> > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > ---
> >  net/sctp/socket.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > index cedd9bf..0a5f2bf 100644
> > --- a/net/sctp/socket.c
> > +++ b/net/sctp/socket.c
> > @@ -5652,6 +5652,8 @@ static int sctp_getsockopt_assoc_stats(struct sock *sk, int len,
> >  	/* User must provide at least the assoc id */
> >  	if (len < sizeof(sctp_assoc_t))
> >  		return -EINVAL;
> > +	if (len > sizeof(struct sctp_assoc_stats))
> > +		len = sizeof(struct sctp_assoc_stats);
> >  
> >  	if (copy_from_user(&sas, optval, len))
> >  		return -EFAULT;
> > -- 
> > 1.7.9.7
> > 
> > 
> 
> Theres more than that going on here.  This will fix the warning, but the
> function is written such that, if you pass in a size that is greater than the
> size of a struct sctp_association, but less than a struct sctp_assoc_stats.  I'm
> not sure that a partial stat struct is really that useful to people.  What if
> you were to check for max(struct sctp_association, struct sctp_assoc_stats) as
> your minimum length check, then just did a copy_from_user of that length.  It
> would save you having to compute two lengths separately, since you could then
> just do a copy_to_user(...,sizeof(struct sctp_assoc_stats), at the bottom of
> that function.
> 
Yes, but that would require input from someone who knows the code. All I am trying
to accomplish is to ensure that copy_from_user does not overwrite the stack.

Thanks,
Guenter
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