[RFC net-next PATCH 1/2] bpf: avoid clear xdp_frame area again

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

Avoid clearing xdp_frame area if this was already done by prevous
invocations of bpf_xdp_adjust_head.

The xdp_adjust_head helper can be called multiple times by the
bpf_prog.  If increasing the packet header size (with a negative
offset), kernel must assume bpf_prog store valuable information here,
and not clear this information.

In case of extending header into xdp_frame area the kernel clear this
area to avoid any info leaking.

The bug in the current implementation is that if existing xdp->data
pointer have already been moved into xdp_frame area, then memory is
cleared between new-data pointer and xdp_frame-end, which covers an
area that might contain information store by BPF-prog (as curr
xdp->data lays between those pointers).

Fixes: 6dfb970d3dbd ("xdp: avoid leaking info stored in frame data on page reuse")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/filter.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index a374b8560bc4..15e9b5477360 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2705,6 +2705,13 @@ BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, int, offset)
 	if (data < xdp_frame_end) {
 		unsigned long clearlen = xdp_frame_end - data;
 
+		/* Handle if prev call adjusted xdp->data into xdp_frame area */
+		if (unlikely(xdp->data < xdp_frame_end)) {
+			if (data < xdp->data)
+				clearlen = xdp->data - data;
+			else
+				clearlen = 0;
+		}
 		memset(data, 0, clearlen);
 	}