[PATCH net 6/6] net: stmmac: fix XDP RX buffer DMA sync length

[Lorenzo Bianconi]

Since commit 88095728511e ("net: stmmac: fix rx Scatter-Gather support")
moved the page_pool DMA window to offset 0, the sync range no longer
starts at the XDP headroom. The XDP path still computed sync_len as

        data_end - data_hard_start - buf->page_offset

so page_pool_dma_sync_for_device() synced [base, base + len) instead of
[base + page_offset, base + page_offset + len).

On a non-coherent platform an XDP program can write packet data, write
metadata or grow the tail. If the program then returns XDP_DROP/ABORTED,
or a redirect fails, the driver recycles the head page with an incomplete
sync. Dirty cache lines in the tail of the frame can be written back
after the device has DMA'd a new frame into that page.

Include buf->page_offset in the sync length so the whole CPU-touched
window is synced before the page is reused.

Fixes: 88095728511e ("net: stmmac: fix rx Scatter-Gather support")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi at oss.qualcomm.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 32b2c5834c34..c51358c51fae 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -5980,8 +5980,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 			xdp_prepare_buff(&ctx.xdp, page_address(buf->page),
 					 buf->page_offset, buf1_len, true);
 
-			pre_len = ctx.xdp.data_end - ctx.xdp.data_hard_start -
-				  buf->page_offset;
+			pre_len = ctx.xdp.data_end - ctx.xdp.data_hard_start;
 
 			ctx.priv = priv;
 			ctx.desc = p;
@@ -5991,8 +5990,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 			/* Due xdp_adjust_tail: DMA sync for_device
 			 * cover max len CPU touch
 			 */
-			sync_len = ctx.xdp.data_end - ctx.xdp.data_hard_start -
-				   buf->page_offset;
+			sync_len = ctx.xdp.data_end - ctx.xdp.data_hard_start;
 			sync_len = max(sync_len, pre_len);
 
 			/* For Not XDP_PASS verdict */

-- 
2.56.0