Subject: Re: [PATCH bpf] bpf: fix null pointer deref in bpf_prog_test_run_xdp

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 31 Jan 2018 08:24:03 +0100

On Wed, 31 Jan 2018 01:31:11 +0100
Daniel Borkmann <[email-masked]> wrote:

> syzkaller was able to generate the following XDP program ...
> 
>   (18) r0 = 0x0
>   (61) r5 = *(u32 *)(r1 +12)
>   (04) (u32) r0 += (u32) 0
>   (95) exit
> 
> ... and trigger a NULL pointer dereference in ___bpf_prog_run()
> via bpf_prog_test_run_xdp() where this was attempted to run.
> 
> Reason is that recent xdp_rxq_info addition to XDP programs
> updated all drivers, but not bpf_prog_test_run_xdp(), where
> xdp_buff is set up. Thus when context rewriter does the deref
> on the netdev it's NULL at runtime. Fix it by using xdp_rxq
> from loopback dev. netif_get_rx_queue() helper can also be
> reused in various other locations later on.
> 
> Fixes: 02dd3291b2f0 ("bpf: finally expose xdp_rxq_info to XDP bpf-programs")
> Reported-by: [email-masked]
> Signed-off-by: Daniel Borkmann <[email-masked]>
> Cc: Jesper Dangaard Brouer <[email-masked]>
> ---
>  [ Note: Needs to wait till Linus pulled Dave's net-next so
>    the affected commit lands in bpf tree. ]
> 
>  include/linux/netdevice.h                   |  6 ++++++
>  net/bpf/test_run.c                          |  4 ++++
>  tools/testing/selftests/bpf/test_verifier.c | 14 ++++++++++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index cd46d3d..9630f4e 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3228,6 +3228,12 @@ static inline int netif_set_real_num_rx_queues(struct net_device *dev,
>  }
>  #endif
>  
> +static inline struct netdev_rx_queue *
> +netif_get_rx_queue(struct net_device *dev, unsigned int rxq)

This function name is very close to (net/core/dev.c):

 static struct netdev_rx_queue *
 netif_get_rxqueue(struct sk_buff *skb)

> +{
> +	return dev->_rx + rxq;
> +}

I know above is correct, is just annoys my eyes as I find it more
natural to write as dev->_rx[rxq].  I'm not saying you should change
it, because it does preserve the access style elsewhere (and I also
kept this style in some of my changes).

You mention netif_get_rx_queue() helper can also be reused in various
other locations later on, but there is not boundary checks (on this
dynamically allocated array of size dev->num_rx_queues).  We could add
a comment saying caller is responsible for boundary checks, or prefix
the function name with "__" to indicate this indirectly.

>  #ifdef CONFIG_SYSFS
>  static inline unsigned int get_netdev_rx_queue_index(
>  		struct netdev_rx_queue *queue)
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index a86e668..47cd0b7 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -151,6 +151,7 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>  {
>  	u32 size = kattr->test.data_size_in;
>  	u32 repeat = kattr->test.repeat;
> +	struct netdev_rx_queue *rxqueue;
>  	struct xdp_buff xdp = {};
>  	u32 retval, duration;
>  	void *data;
> @@ -165,6 +166,9 @@ int bpf_prog_test_run_xdp(struct bpf_prog *prog, const union bpf_attr *kattr,
>  	xdp.data_meta = xdp.data;
>  	xdp.data_end = xdp.data + size;
>  
> +	rxqueue = netif_get_rx_queue(current->nsproxy->net_ns->loopback_dev, 0);
> +	xdp.rxq = &rxqueue->xdp_rxq;
> +
>  	retval = bpf_test_run(prog, &xdp, repeat, &duration);
>  	if (xdp.data != data + XDP_PACKET_HEADROOM + NET_IP_ALIGN)
>  		size = xdp.data_end - xdp.data;
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index 697bd83..c0f16e9 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -7780,6 +7780,20 @@ static struct bpf_test tests[] = {
>  		.result = REJECT,
>  	},
>  	{
> +		"XDP, using ifindex from netdev",
> +		.insns = {
> +			BPF_MOV64_IMM(BPF_REG_0, 0),
> +			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
> +				    offsetof(struct xdp_md, ingress_ifindex)),
> +			BPF_JMP_IMM(BPF_JLT, BPF_REG_2, 1, 1),
> +			BPF_MOV64_IMM(BPF_REG_0, 1),
> +			BPF_EXIT_INSN(),
> +		},
> +		.result = ACCEPT,
> +		.prog_type = BPF_PROG_TYPE_XDP,
> +		.retval = 1,
> +	},
> +	{
>  		"meta access, test1",
>  		.insns = {
>  			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer