[PATCH 5.0 046/122] xdp: fix cpumap redirect SKB creation bug

From: Greg Kroah-Hartman
Date: Mon May 06 2019 - 10:37:29 EST

[ Upstream commit 676e4a6fe703f2dae699ee9d56f14516f9ada4ea ]

We want to avoid leaking pointer info from xdp_frame (that is placed in
top of frame) like commit 6dfb970d3dbd ("xdp: avoid leaking info stored in
frame data on page reuse"), and followup commit 97e19cce05e5 ("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@xxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Sasha Levin (Microsoft) <sashal@xxxxxxxxxx>
kernel/bpf/cpumap.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index 8974b3755670..3c18260403dd 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -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;