[PATCH net-next] net: stmmac: ptp: limit n_per_out

["Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>]

ptp_clock_ops.n_per_out sets the number of PPS outputs, which the PTP
subsystem uses to validate userspace input, such as the index number
used in a PTP_CLK_REQ_PEROUT request.

stmmac_enable() uses this to index the priv->pps array, which is an
array of size STMMAC_PPS_MAX. ptp_clock_ops.n_per_out is initialised
using priv->dma_cap.pps_out_num, which is a three bit field read from
hardware.

Documentation that I've checked suggests that values >= 5 are reserved,
but that doesn't mean such values won't appear, and if they do, we
can overrun the priv->pps array in stmmac_enable().

stmmac_ptp_register() has protection against this in its loop, but it
doesn't act to limit ptp_clock_ops.n_per_out.

Fix this by introducing a local variable, pps_out_num which is limited
to STMMAC_PPS_MAX, and use that when initialising the array and setting
priv->ptp_clock_ops.n_per_out.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
---

This could be a user exploitable bug (although one has to be root
so the gun is already pointing at one's foot.) This is the commit
which introduced the problem:

Fixes: 9a8a02c9d46d ("net: stmmac: Add Flexible PPS support")

 drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
index 3e30172fa129..cf5506bf2198 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
@@ -334,13 +334,14 @@ const struct ptp_clock_info dwmac1000_ptp_clock_ops = {
  */
 void stmmac_ptp_register(struct stmmac_priv *priv)
 {
+	unsigned int pps_out_num = priv->dma_cap.pps_out_num;
 	int i;
 
-	for (i = 0; i < priv->dma_cap.pps_out_num; i++) {
-		if (i >= STMMAC_PPS_MAX)
-			break;
+	if (pps_out_num > STMMAC_PPS_MAX)
+		pps_out_num = STMMAC_PPS_MAX;
+
+	for (i = 0; i < pps_out_num; i++)
 		priv->pps[i].available = true;
-	}
 
 	/* Calculate the clock domain crossing (CDC) error if necessary */
 	priv->plat->cdc_error_adj = 0;
@@ -350,8 +351,8 @@ void stmmac_ptp_register(struct stmmac_priv *priv)
 	/* Update the ptp clock parameters based on feature discovery, when
 	 * available
 	 */
-	if (priv->dma_cap.pps_out_num)
-		priv->ptp_clock_ops.n_per_out = priv->dma_cap.pps_out_num;
+	if (pps_out_num)
+		priv->ptp_clock_ops.n_per_out = pps_out_num;
 
 	if (priv->dma_cap.aux_snapshot_n)
 		priv->ptp_clock_ops.n_ext_ts = priv->dma_cap.aux_snapshot_n;
-- 
2.47.3