net: stmmac: do not keep the new TSO MSS cached on mapping failures

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stmmac_tso_xmit() fills the MSS context descriptor and stores the new MSS
in tx_q->mss right away, but the descriptor only gets its OWN bit much
later, right before the frame is handed to the DMA. Every error path in
between - the dma_map_single() of the linear part and the
skb_frag_dma_map() of each fragment - returns with tx_q->mss already
updated while the MAC is still programmed with the previous MSS.

The context descriptor is now handled like the data descriptors are:
tx_q->cur_tx is not advanced while it is being filled, so the slot stays
where the next transmit fills it, and the error paths release it
explicitly instead of leaving a descriptor the DMA will stop on in the
middle of the ring. With the queue no longer wedged by that descriptor,
the stale cached MSS is what remains: the next skb carrying the same
gso_size compares equal to the cached value, no context descriptor is
emitted, and the hardware segments the TCP stream with the MSS of an
earlier frame, generating frames whose payload size does not match what
the stack accounted for.

Drop the cached value on those paths instead. Zero is what
stmmac_reset_tx_queue() leaves behind, so the next TSO frame programs the
context descriptor again.

The store itself stays ahead of the mappings, as before, rather than
moving next to the OWN bit: stmmac_tx_err() reinitialises the channel and
clears tx_q->mss from the DMA interrupt handler, which takes no TX queue
lock, so it can run in the middle of stmmac_tso_xmit(). Publishing the
cache after the descriptor has been handed over would widen the window in
which such a reset is overwritten with an MSS that the reinitialised
channel was never programmed with.

Found by manual code inspection and AI tools.
Not triggered or tested on real hardware.

Cc: stable+noautosel@kernel.org # untested fix to unlikely driver error path
Fixes: f748be531d ("stmmac: support new GMAC4")
Signed-off-by: Linkui Xiao <xiaolinkui@kylinos.cn>
Link: https://patch.msgid.link/20260928123422.1698785-1-xiaolinkui@126.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

...

This commit is contained in:

Linkui Xiao 2026-09-28 20:34:22 +08:00 • committed by Jakub Kicinski

parent 0ac396fbc2

commit 6b76e4efe7

1 changed files with 13 additions and 3 deletions

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

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