Subject: [PATCH] net: stmmac: Avoid VLA usage

From: Kees Cook <[email-masked]>
Date: Tue, 1 May 2018 14:01:30 -0700

In the quest to remove all stack VLAs from the kernel[1], this switches
the "status" stack buffer to use the existing small (8) upper bound on
how many queues can be checked for DMA, and adds a sanity-check just to
make sure it doesn't operate under pathological conditions.

[1] http://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=[email-masked]

Signed-off-by: Kees Cook <[email-masked]>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index b65e2d144698..19bdc23fa314 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -2045,7 +2045,11 @@ static void stmmac_dma_interrupt(struct stmmac_priv *priv)
 				tx_channel_count : rx_channel_count;
 	u32 chan;
 	bool poll_scheduled = false;
-	int status[channels_to_check];
+	int status[max_t(u32, MTL_MAX_TX_QUEUES, MTL_MAX_RX_QUEUES)];
+
+	/* Make sure we never check beyond our status buffer. */
+	if (WARN_ON_ONCE(channels_to_check > ARRAY_SIZE(status)))
+		channels_to_check = ARRAY_SIZE(status);
 
 	/* Each DMA channel can be used for rx and tx simultaneously, yet
 	 * napi_struct is embedded in struct stmmac_rx_queue rather than in a
-- 
2.7.4

-- 
Kees Cook
Pixel Security