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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@gmail.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@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.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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