[PATCH net v2] net: stmmac: fix integer underflow in chain mode

Tyllis Xu
2026-04-01T04:47:50Z

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>
---
v2: Instead of restructuring into an if/else block (v1), introduce a
buf_len local variable clamped to min(nopaged_len, bmax) so the
subtraction and the existing while loop remain structurally intact.
Suggested by Russell King.

drivers/net/ethernet/stmicro/stmmac/chain_mode.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/chain_mode.c b/drivers/net/ethernet/stmicro/stmmac/chain_mode.c
index 120a009c9992..37f9417c7c0e 100644
--- 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) {
--
2.43.0