Commit 7d8677ff authored Mar 29, 2019 by Jesper Dangaard Brouer Committed by Greg Kroah-Hartman Dec 05, 2019

xdp: fix cpumap redirect SKB creation bug

[ Upstream commit 676e4a6fe703f2dae699ee9d56f14516f9ada4ea ]

We want to avoid leaking pointer info from xdp_frame (that is placed in
top of frame) like commit 6dfb970d ("xdp: avoid leaking info stored in
frame data on page reuse"), and followup commit 97e19cce ("bpf:
reserve xdp_frame size in xdp headroom") that reserve this headroom.

These changes also affected how cpumap constructed SKBs, as xdpf->headroom
size changed, the skb data starting point were in-effect shifted with 32
bytes (sizeof xdp_frame). This was still okay, as the cpumap frame_size
calculation also included xdpf->headroom which were reduced by same amount.

A bug was introduced in commit 77ea5f4cbe20 ("bpf/cpumap: make sure
frame_size for build_skb is aligned if headroom isn't"), where the
xdpf->headroom became part of the SKB_DATA_ALIGN rounding up. This
round-up to find the frame_size is in principle still correct as it does
not exceed the 2048 bytes frame_size (which is max for ixgbe and i40e),
but the 32 bytes offset of pkt_data_start puts this over the 2048 bytes
limit. This cause skb_shared_info to spill into next frame. It is a little
hard to trigger, as the SKB need to use above 15 skb_shinfo->frags[] as
far as I calculate. This does happen in practise for TCP streams when
skb_try_coalesce() kicks in.

KASAN can be used to detect these wrong memory accesses, I've seen:
BUG: KASAN: use-after-free in skb_try_coalesce+0x3cb/0x760
BUG: KASAN: wild-memory-access in skb_release_data+0xe2/0x250

Driver veth also construct a SKB from xdp_frame in this way, but is not
affected, as it doesn't reserve/deduct the room (used by xdp_frame) from
the SKB headroom. Instead is clears the pointers via xdp_scrub_frame(),
and allows SKB to use this area.

The fix in this patch is to do like veth and instead allow SKB to (re)use
the area occupied by xdp_frame, by clearing via xdp_scrub_frame(). (This
does kill the idea of the SKB being able to access (mem) info from this
area, but I guess it was a bad idea anyhow, and it was already killed by
the veth changes.)

Fixes: 77ea5f4cbe20 ("bpf/cpumap: make sure frame_size for build_skb is aligned if headroom isn't")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 32a24a39

kernel/bpf/cpumap.c

+10 −3

Original line number	Diff line number	Diff line
		@@ -162,10 +162,14 @@ static void cpu_map_kthread_stop(struct work_struct *work)
		static struct sk_buff cpu_map_build_skb(struct bpf_cpu_map_entry rcpu,
		struct xdp_frame *xdpf)
		{
		unsigned int hard_start_headroom;
		unsigned int frame_size;
		void *pkt_data_start;
		struct sk_buff *skb;

		/* Part of headroom was reserved to xdpf */
		hard_start_headroom = sizeof(struct xdp_frame) + xdpf->headroom;

		/* build_skb need to place skb_shared_info after SKB end, and
		* also want to know the memory "truesize". Thus, need to
		* know the memory frame size backing xdp_buff.
		@@ -183,15 +187,15 @@ static struct sk_buff cpu_map_build_skb(struct bpf_cpu_map_entry rcpu,
		* is not at a fixed memory location, with mixed length
		* packets, which is bad for cache-line hotness.
		*/
		frame_size = SKB_DATA_ALIGN(xdpf->len + xdpf->headroom) +
		frame_size = SKB_DATA_ALIGN(xdpf->len + hard_start_headroom) +
		SKB_DATA_ALIGN(sizeof(struct skb_shared_info));

		pkt_data_start = xdpf->data - xdpf->headroom;
		pkt_data_start = xdpf->data - hard_start_headroom;
		skb = build_skb(pkt_data_start, frame_size);
		if (!skb)
		return NULL;

		skb_reserve(skb, xdpf->headroom);
		skb_reserve(skb, hard_start_headroom);
		__skb_put(skb, xdpf->len);
		if (xdpf->metasize)
		skb_metadata_set(skb, xdpf->metasize);
		@@ -205,6 +209,9 @@ static struct sk_buff cpu_map_build_skb(struct bpf_cpu_map_entry rcpu,
		* - RX ring dev queue index (skb_record_rx_queue)
		*/

		/* Allow SKB to reuse area used by xdp_frame */
		xdp_scrub_frame(xdpf);

		return skb;
		}