Re: [PATCH v2] net: sched: fix memory leak in tcindex_set_parms

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2022-11-14 at 01:05 +0800, Hawkins Jiawei wrote:
> Syzkaller reports a memory leak as follows:
> ====================================
> BUG: memory leak
> unreferenced object 0xffff88810c287f00 (size 256):
>   comm "syz-executor105", pid 3600, jiffies 4294943292 (age 12.990s)
>   hex dump (first 32 bytes):
>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>   backtrace:
>     [<ffffffff814cf9f0>] kmalloc_trace+0x20/0x90 mm/slab_common.c:1046
>     [<ffffffff839c9e07>] kmalloc include/linux/slab.h:576 [inline]
>     [<ffffffff839c9e07>] kmalloc_array include/linux/slab.h:627 [inline]
>     [<ffffffff839c9e07>] kcalloc include/linux/slab.h:659 [inline]
>     [<ffffffff839c9e07>] tcf_exts_init include/net/pkt_cls.h:250 [inline]
>     [<ffffffff839c9e07>] tcindex_set_parms+0xa7/0xbe0 net/sched/cls_tcindex.c:342
>     [<ffffffff839caa1f>] tcindex_change+0xdf/0x120 net/sched/cls_tcindex.c:553
>     [<ffffffff8394db62>] tc_new_tfilter+0x4f2/0x1100 net/sched/cls_api.c:2147
>     [<ffffffff8389e91c>] rtnetlink_rcv_msg+0x4dc/0x5d0 net/core/rtnetlink.c:6082
>     [<ffffffff839eba67>] netlink_rcv_skb+0x87/0x1d0 net/netlink/af_netlink.c:2540
>     [<ffffffff839eab87>] netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
>     [<ffffffff839eab87>] netlink_unicast+0x397/0x4c0 net/netlink/af_netlink.c:1345
>     [<ffffffff839eb046>] netlink_sendmsg+0x396/0x710 net/netlink/af_netlink.c:1921
>     [<ffffffff8383e796>] sock_sendmsg_nosec net/socket.c:714 [inline]
>     [<ffffffff8383e796>] sock_sendmsg+0x56/0x80 net/socket.c:734
>     [<ffffffff8383eb08>] ____sys_sendmsg+0x178/0x410 net/socket.c:2482
>     [<ffffffff83843678>] ___sys_sendmsg+0xa8/0x110 net/socket.c:2536
>     [<ffffffff838439c5>] __sys_sendmmsg+0x105/0x330 net/socket.c:2622
>     [<ffffffff83843c14>] __do_sys_sendmmsg net/socket.c:2651 [inline]
>     [<ffffffff83843c14>] __se_sys_sendmmsg net/socket.c:2648 [inline]
>     [<ffffffff83843c14>] __x64_sys_sendmmsg+0x24/0x30 net/socket.c:2648
>     [<ffffffff84605fd5>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>     [<ffffffff84605fd5>] do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>     [<ffffffff84800087>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
> ====================================
> 
> Kernel uses tcindex_change() to change an existing
> traffic-control-indices filter properties. During the
> process of changing, kernel clears the old
> traffic-control-indices filter result, and updates it
> by RCU assigning new traffic-control-indices data.
> 
> Yet the problem is that, kernel clears the old
> traffic-control-indices filter result, without destroying
> its tcf_exts structure, which triggers the above
> memory leak.
> 
> This patch solves it by using tcf_exts_destroy() to
> destroy the tcf_exts structure in old
> traffic-control-indices filter result, after the
> RCU grace period.
> 
> [Thanks to the suggestion from Jakub Kicinski and Cong Wang]
> 
> Fixes: b9a24bb76bf6 ("net_sched: properly handle failure case of tcf_exts_init()")
> Link: https://lore.kernel.org/all/0000000000001de5c505ebc9ec59@google.com/
> Reported-by: syzbot+232ebdbd36706c965ebf@syzkaller.appspotmail.com
> Tested-by: syzbot+232ebdbd36706c965ebf@syzkaller.appspotmail.com
> Cc: Cong Wang <cong.wang@bytedance.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
> ---
> v2:
>   - remove all 'will' in commit message according to Jakub Kicinski
>   - add Fixes tag according to Jakub Kicinski
>   - remove all ifdefs according to Jakub Kicinski and Cong Wang
>   - add synchronize_rcu() before destorying old_e according to
> Cong Wang
> 
> v1: https://lore.kernel.org/all/20221031060835.11722-1-yin31149@gmail.com/
>  net/sched/cls_tcindex.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/sched/cls_tcindex.c b/net/sched/cls_tcindex.c
> index 1c9eeb98d826..d2fac9559d3e 100644
> --- a/net/sched/cls_tcindex.c
> +++ b/net/sched/cls_tcindex.c
> @@ -338,6 +338,7 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
>  	struct tcf_result cr = {};
>  	int err, balloc = 0;
>  	struct tcf_exts e;
> +	struct tcf_exts old_e = {};
>  
>  	err = tcf_exts_init(&e, net, TCA_TCINDEX_ACT, TCA_TCINDEX_POLICE);
>  	if (err < 0)
> @@ -479,6 +480,7 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
>  	}
>  
>  	if (old_r && old_r != r) {
> +		old_e = old_r->exts;
>  		err = tcindex_filter_result_init(old_r, cp, net);
>  		if (err < 0) {
>  			kfree(f);
> @@ -510,6 +512,12 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
>  		tcf_exts_destroy(&new_filter_result.exts);
>  	}
>  
> +	/* Note: old_e should be destroyed after the RCU grace period,
> +	 * to avoid possible use-after-free by concurrent readers.
> +	 */
> +	synchronize_rcu();

this could make tc reconfiguration potentially very slow. I'm wondering
if we can delegate the tcf_exts_destroy() to some workqueue?

Thanks!

Paolo