[net-next V11 PATCH 17/17] xdp: avoid leaking info stored in frame data on page reuse

[Jesper Dangaard Brouer <brouer@redhat.com>]

The bpf infrastructure and verifier goes to great length to avoid
bpf progs leaking kernel (pointer) info.

For queueing an xdp_buff via XDP_REDIRECT, xdp_frame info stores
kernel info (incl pointers) in top part of frame data (xdp->data_hard_start).
Checks are in place to assure enough headroom is available for this.

This info is not cleared, and if the frame is reused, then a
malicious user could use bpf_xdp_adjust_head helper to move
xdp->data into this area.  Thus, making this area readable.

This is not super critical as XDP progs requires root or
CAP_SYS_ADMIN, which are privileged enough for such info.  An
effort (is underway) towards moving networking bpf hooks to the
lesser privileged mode CAP_NET_ADMIN, where leaking such info
should be avoided.  Thus, this patch to clear the info when
needed.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/filter.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 3bb0cb98a9be..a374b8560bc4 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2692,6 +2692,7 @@ static unsigned long xdp_get_metalen(const struct xdp_buff *xdp)
 
 BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, int, offset)
 {
+	void *xdp_frame_end = xdp->data_hard_start + sizeof(struct xdp_frame);
 	unsigned long metalen = xdp_get_metalen(xdp);
 	void *data_start = xdp->data_hard_start + metalen;
 	void *data = xdp->data + offset;
@@ -2700,6 +2701,13 @@ BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, int, offset)
 		     data > xdp->data_end - ETH_HLEN))
 		return -EINVAL;
 
+	/* Avoid info leak, when reusing area prev used by xdp_frame */
+	if (data < xdp_frame_end) {
+		unsigned long clearlen = xdp_frame_end - data;
+
+		memset(data, 0, clearlen);
+	}
+
 	if (metalen)
 		memmove(xdp->data_meta + offset,
 			xdp->data_meta, metalen);