Re: [PATCH v2 1/2] sctp: export sctp_endpoint_{hold,put}() and return incremented endpoint

From: Jakub Kicinski <kuba@kernel.org>
To: Lee Jones <lee.jones@linaro.org>
Cc: linux-kernel@vger.kernel.org, Vlad Yasevich <vyasevich@gmail.com>,
	Neil Horman <nhorman@tuxdriver.com>,
	Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	lksctp developers <linux-sctp@vger.kernel.org>,
	"H.P. Yarroll" <piggy@acm.org>,
	Karl Knutson <karl@athena.chicago.il.us>,
	Jon Grimm <jgrimm@us.ibm.com>,
	Xingang Guo <xingang.guo@intel.com>,
	Hui Huang <hui.huang@nokia.com>,
	Sridhar Samudrala <sri@us.ibm.com>,
	Daisy Chang <daisyc@us.ibm.com>, Ryan Layer <rmlayer@us.ibm.com>,
	Kevin Gao <kevin.gao@intel.com>,
	netdev@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] sctp: export sctp_endpoint_{hold,put}() and return incremented endpoint
Date: Fri, 17 Dec 2021 07:06:26 -0800	[thread overview]
Message-ID: <20211217070626.790b8340@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20211217134607.74983-1-lee.jones@linaro.org>

On Fri, 17 Dec 2021 13:46:06 +0000 Lee Jones wrote:
> For example, in sctp_sock_dump(), we could have the following hunk:
> 
> 	sctp_endpoint_hold(tsp->asoc->ep);
> 	ep = tsp->asoc->ep;
> 	sk = ep->base.sk
> 	lock_sock(ep->base.sk);
> 
> It is possible for this task to be swapped out immediately following
> the call into sctp_endpoint_hold() that would change the address of
> tsp->asoc->ep to point to a completely different endpoint.  This means
> a reference could be taken to the old endpoint and the new one would
> be processed without a reference taken, moreover the new endpoint
> could then be freed whilst still processing as a result, causing a
> use-after-free.
> 
> If we return the exact pointer that was held, we ensure this task
> processes only the endpoint we have taken a reference to.  The
> resultant hunk now looks like this:
> 
>       ep = sctp_endpoint_hold(tsp->asoc->ep);
> 	sk = ep->base.sk
> 	lock_sock(sk);

If you have to explain what the next patch will do to make sense 
of this one it really is better to merge the two patches.
Exporting something is not a functional change, nor does it make
the changes easier to review, in fact the opposite is true.

> Fixes: 8f840e47f190c ("sctp: add the sctp_diag.c file")

This patch in itself fixes exactly nothing.