CVE-2024-36016

In the Linux kernel, the following vulnerability has been resolved:

tty: n_gsm: fix possible out-of-bounds in gsm0_receive()

Assuming the following:

• side A configures the n_gsm in basic option mode
• side B sends the header of a basic option mode frame with data length 1
• side A switches to advanced option mode
• side B sends 2 data bytes which exceeds gsm->len
  Reason: gsm->len is not used in advanced option mode.
• side A switches to basic option mode
• side B keeps sending until gsm0_receive() writes past gsm->buf
  Reason: Neither gsm->state nor gsm->len have been reset after
  reconfiguration.

Fix this by changing gsm->count to gsm->len comparison from equal to less
than. Also add upper limit checks against the constant MAX_MRU in
gsm0_receive() and gsm1_receive() to harden against memory corruption of
gsm->len and gsm->mru.

All other checks remain as we still need to limit the data according to the
user configuration and actual payload size.