linux-kernel - [20/55] tty/n_gsm: fix bug in CRC calculation for gsm1 mode

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Message-Id: <20110429185654.515303419@clark.kroah.org>
Date:	Fri, 29 Apr 2011 11:55:43 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Mikhail Kshevetskiy <mikhail.kshevetskiy@...il.com>,
	Alan Cox <alan@...ux.intel.com>
Subject: [20/55] tty/n_gsm: fix bug in CRC calculation for gsm1 mode

2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Mikhail Kshevetskiy <mikhail.kshevetskiy@...il.com>

commit 9db4e4381a8e881ff65a5d3400bfa471f84217e7 upstream.

Problem description:
  gsm_queue() calculate a CRC for arrived frames. As a last step of
  CRC calculation it call

    gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);

  This work perfectly for the case of GSM0 mode as gsm->received_fcs
  contain the last piece of data required to generate final CRC.

  gsm->received_fcs is not used for GSM1 mode. Thus we put an
  additional byte to CRC calculation. As result we get a wrong CRC
  and reject incoming frame.

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@...il.com>
Acked-by: Alan Cox <alan@...ux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/tty/n_gsm.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1659,8 +1659,12 @@ static void gsm_queue(struct gsm_mux *gs
 
 	if ((gsm->control & ~PF) == UI)
 		gsm->fcs = gsm_fcs_add_block(gsm->fcs, gsm->buf, gsm->len);
-	/* generate final CRC with received FCS */
-	gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);
+	if (gsm->encoding == 0){
+		/* WARNING: gsm->received_fcs is used for gsm->encoding = 0 only.
+		            In this case it contain the last piece of data
+		            required to generate final CRC */
+		gsm->fcs = gsm_fcs_add(gsm->fcs, gsm->received_fcs);
+	}
 	if (gsm->fcs != GOOD_FCS) {
 		gsm->bad_fcs++;
 		if (debug & 4)


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