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: 286a83721720 ("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>

diff --git a/drivers/net/ethernet/stmicro/stmmac/chain_mode.c b/drivers/net/ethernet/stmicro/stmmac/chain_mode.c
index 120a009c99929914049ebe7bef42644577f56392..37f9417c7c0e04f4342393e142d5b55516407d0f 100644 (file)
--- a/drivers/net/ethernet/stmicro/stmmac/chain_mode.c
+++ b/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) {