| From 8838b2af23caf1ff0610caef2795d6668a013b2d Mon Sep 17 00:00:00 2001 |
| From: "daniel.starke@siemens.com" <daniel.starke@siemens.com> |
| Date: Thu, 20 Jan 2022 02:18:57 -0800 |
| Subject: tty: n_gsm: fix SW flow control encoding/handling |
| |
| From: daniel.starke@siemens.com <daniel.starke@siemens.com> |
| |
| commit 8838b2af23caf1ff0610caef2795d6668a013b2d upstream. |
| |
| n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010. |
| See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516 |
| The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to |
| the newer 27.010 here. Chapter 5.2.7.3 states that DC1 (XON) and DC3 (XOFF) |
| are the control characters defined in ISO/IEC 646. These shall be quoted if |
| seen in the data stream to avoid interpretation as flow control characters. |
| |
| ISO/IEC 646 refers to the set of ISO standards described as the ISO |
| 7-bit coded character set for information interchange. Its final version |
| is also known as ITU T.50. |
| See https://www.itu.int/rec/T-REC-T.50-199209-I/en |
| |
| To abide the standard it is needed to quote DC1 and DC3 correctly if these |
| are seen as data bytes and not as control characters. The current |
| implementation already tries to enforce this but fails to catch all |
| defined cases. 3GPP 27.010 chapter 5.2.7.3 clearly states that the most |
| significant bit shall be ignored for DC1 and DC3 handling. The current |
| implementation handles only the case with the most significant bit set 0. |
| Cases in which DC1 and DC3 have the most significant bit set 1 are left |
| unhandled. |
| |
| This patch fixes this by masking the data bytes with ISO_IEC_646_MASK (only |
| the 7 least significant bits set 1) before comparing them with XON |
| (a.k.a. DC1) and XOFF (a.k.a. DC3) when testing which byte values need |
| quotation via byte stuffing. |
| |
| Fixes: e1eaea46bb40 ("tty: n_gsm line discipline") |
| Cc: stable@vger.kernel.org |
| Signed-off-by: Daniel Starke <daniel.starke@siemens.com> |
| Link: https://lore.kernel.org/r/20220120101857.2509-1-daniel.starke@siemens.com |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| drivers/tty/n_gsm.c | 4 +++- |
| 1 file changed, 3 insertions(+), 1 deletion(-) |
| |
| --- a/drivers/tty/n_gsm.c |
| +++ b/drivers/tty/n_gsm.c |
| @@ -322,6 +322,7 @@ static int addr_cnt; |
| #define GSM1_ESCAPE_BITS 0x20 |
| #define XON 0x11 |
| #define XOFF 0x13 |
| +#define ISO_IEC_646_MASK 0x7F |
| |
| static const struct tty_port_operations gsm_port_ops; |
| |
| @@ -531,7 +532,8 @@ static int gsm_stuff_frame(const u8 *inp |
| int olen = 0; |
| while (len--) { |
| if (*input == GSM1_SOF || *input == GSM1_ESCAPE |
| - || *input == XON || *input == XOFF) { |
| + || (*input & ISO_IEC_646_MASK) == XON |
| + || (*input & ISO_IEC_646_MASK) == XOFF) { |
| *output++ = GSM1_ESCAPE; |
| *output++ = *input++ ^ GSM1_ESCAPE_BITS; |
| olen++; |
