[PATCH net-next v6 2/3] net: stmmac: guard against a zero channel in the aux snapshot handler

The generic timestamp_interrupt() handler derives the EXTTS channel
index with ilog2() applied directly to the PTP_ACR channel mask, with
no check for a zero mask.  ilog2(0) yields -1, which ends up in
event.index as 0xffffffff, and ptp_clock_event() uses that index in
test_bit() against a PTP_MAX_CHANNELS bitmap without range validation,
reading far past the allocation from hard IRQ context.

The zero-mask window is reachable: stmmac_enable() sets
STMMAC_FLAG_EXT_SNAPSHOT_EN before it programs PTP_ACR, so an EXTTS
interrupt arriving in between passes the flag check while the mask is
still clear (snapshots can also linger in the FIFO from a previous
enable, as only PTP_ACR_ATSFC clears them).

Check the mask before applying the ilog2().

Fixes: 8851346912a1 ("net: stmmac: Assign configured channel value to EXTTS event")
Cc: stable@vger.kernel.org
Signed-off-by: Zxyan Zhu <redacted>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
index b9a985fa772c..2a076e228e9a 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c

@@ -242,7 +242,10 @@ static void timestamp_interrupt(struct stmmac_priv *priv)
 		       GMAC_TIMESTAMP_ATSNS_SHIFT;
 
 	acr_value = readl(priv->ptpaddr + PTP_ACR);
-	channel = ilog2(FIELD_GET(PTP_ACR_MASK, acr_value));
+	channel = FIELD_GET(PTP_ACR_MASK, acr_value);
+	if (!channel)
+		return;
+	channel = ilog2(channel);
 
 	for (i = 0; i < num_snapshot; i++) {
 		read_lock_irqsave(&priv->ptp_lock, flags);

-- 
2.34.1