[PATCH] net: stmmac: guard FCS stripping against runt frames

stmmac_rx_zc() and stmmac_rx() strip the 4-byte FCS from the last
buffer when ACS (Automatic Checksum Stripping) is disabled:

    buf1_len -= ETH_FCS_LEN;
    len -= ETH_FCS_LEN;

Neither path checks that the buffer actually contains at least
ETH_FCS_LEN bytes. A runt frame delivered by the hardware with a
buf1_len or buf2_len smaller than 4 underflows the unsigned
subtraction, producing a very large value. In the XDP zero-copy path
this wraps data_end backwards:

    buf->xdp->data_end = buf->xdp->data + buf1_len;

giving the XDP/BPF program an enormous data region that extends into
adjacent kernel memory.

Add the missing lower-bound checks so that undersized frames are
silently passed through without stripping.

Fixes: bba2556efad6 ("net: stmmac: Enable RX via AF_XDP zero-copy")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Aldo Ariel Panzardo <redacted>
---

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 1fb5f80..4526669 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5636,7 +5636,8 @@ static int stmmac_rx_zc(struct stmmac_priv *priv, int limit, u32 queue)
 		len += buf1_len;
 
 		/* ACS is disabled; strip manually. */
-		if (likely(!(status & rx_not_ls))) {
+		if (likely(!(status & rx_not_ls)) &&
+		    likely(buf1_len >= ETH_FCS_LEN)) {
 			buf1_len -= ETH_FCS_LEN;
 			len -= ETH_FCS_LEN;
 		}

@@ -5808,10 +5809,10 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 
 		/* ACS is disabled; strip manually. */
 		if (likely(!(status & rx_not_ls))) {
-			if (buf2_len) {
+			if (buf2_len >= ETH_FCS_LEN) {
 				buf2_len -= ETH_FCS_LEN;
 				len -= ETH_FCS_LEN;
-			} else if (buf1_len) {
+			} else if (buf1_len >= ETH_FCS_LEN) {
 				buf1_len -= ETH_FCS_LEN;
 				len -= ETH_FCS_LEN;
 			}

-- 
2.43.0