net: stmmac: fix integer underflow in chain mode

The jumbo_frm() chain-mode implementation unconditionally computes

    len = nopaged_len - bmax;

where nopaged_len = skb_headlen(skb) (linear bytes only) and bmax is
BUF_SIZE_8KiB or BUF_SIZE_2KiB.  However, the caller stmmac_xmit()
decides to invoke jumbo_frm() based on skb->len (total length including
page fragments):

    is_jumbo = stmmac_is_jumbo_frm(priv, skb->len, enh_desc);

When a packet has a small linear portion (nopaged_len <= bmax) but a
large total length due to page fragments (skb->len > bmax), the
subtraction wraps as an unsigned integer, producing a huge len value
(~0xFFFFxxxx).  This causes the while (len != 0) loop to execute
hundreds of thousands of iterations, passing skb->data + bmax * i
pointers far beyond the skb buffer to dma_map_single().  On IOMMU-less
SoCs (the typical deployment for stmmac), this maps arbitrary kernel
memory to the DMA engine, constituting a kernel memory disclosure and
potential memory corruption from hardware.

Fix this by introducing a buf_len local variable clamped to
min(nopaged_len, bmax).  Computing len = nopaged_len - buf_len is then
always safe: it is zero when the linear portion fits within a single
descriptor, causing the while (len != 0) loop to be skipped naturally,
and the fragment loop in stmmac_xmit() handles page fragments afterward.

Fixes: 286a837 ("stmmac: add CHAINED descriptor mode support (V4)")
Cc: stable@vger.kernel.org
Signed-off-by: Tyllis Xu <LivelyCarpet87@gmail.com>
Link: https://patch.msgid.link/20260401044708.1386919-1-LivelyCarpet87@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: LivelyCarpet87

drivers/net/ethernet/stmicro/stmmac/chain_mode.c

@@ -20,7 +20,7 @@ static int jumbo_frm(struct stmmac_tx_queue *tx_q, struct sk_buff *skb,
 	unsigned int nopaged_len = skb_headlen(skb);
 	struct stmmac_priv *priv = tx_q->priv_data;
 	unsigned int entry = tx_q->cur_tx;
-	unsigned int bmax, des2;
+	unsigned int bmax, buf_len, des2;
 	unsigned int i = 1, len;
 	struct dma_desc *desc;
 
@@ -31,17 +31,18 @@ static int jumbo_frm(struct stmmac_tx_queue *tx_q, struct sk_buff *skb,
 	else
 		bmax = BUF_SIZE_2KiB;
 
-	len = nopaged_len - bmax;
+	buf_len = min_t(unsigned int, nopaged_len, bmax);
+	len = nopaged_len - buf_len;
 
 	des2 = dma_map_single(priv->device, skb->data,
-			      bmax, DMA_TO_DEVICE);
+			      buf_len, DMA_TO_DEVICE);
 	desc->des2 = cpu_to_le32(des2);
 	if (dma_mapping_error(priv->device, des2))
 		return -1;
 	tx_q->tx_skbuff_dma[entry].buf = des2;
-	tx_q->tx_skbuff_dma[entry].len = bmax;
+	tx_q->tx_skbuff_dma[entry].len = buf_len;
 	/* do not close the descriptor and do not set own bit */
-	stmmac_prepare_tx_desc(priv, desc, 1, bmax, csum, STMMAC_CHAIN_MODE,
+	stmmac_prepare_tx_desc(priv, desc, 1, buf_len, csum, STMMAC_CHAIN_MODE,
 			0, false, skb->len);
 
 	while (len != 0) {