FAILED: patch "[PATCH] tty: n_gsm: fix buffer over-read in gsm_dlci_data()" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From fd442e5ba30aaa75ea47b32149e7a3110dc20a46 Mon Sep 17 00:00:00 2001
From: Daniel Starke <[email protected]>
Date: Wed, 4 May 2022 10:17:31 +0200
Subject: [PATCH] tty: n_gsm: fix buffer over-read in gsm_dlci_data()

'len' is decreased after each octet that has its EA bit set to 0, which means that the value is encoded with additional octets. However, the final octet does not decreases 'len' which results in 'len' being one byte too long. A buffer over-read may occur in tty_insert_flip_string() as it tries to read one byte more than the passed content size of 'data'. Decrease 'len' also for the final octet which has the EA bit set to 1 to write the correct number of bytes from the internal receive buffer to the virtual tty.

index a38b922bcbc1..9b0b435cf26e 100644
@@ -1658,6 +1658,7 @@ static void gsm_dlci_data(struct gsm_dlci *dlci, const u8 *data, int clen)
 			if (len == 0)
 				return;
 		}
+		len--;
 		slen++;
 		tty = tty_port_tty_get(port);
 		if (tty) {