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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/9513d4148950b05bc99fa7314dc883cc0e1605e5
• https://git.kernel.org/stable/c/b229bc6c6ea9fe459fc3fa94fd0a27a2f32aca56
• https://git.kernel.org/stable/c/0fb736c9931e02dbc7d9a75044c8e1c039e50f04
• https://git.kernel.org/stable/c/4c267110fc110390704cc065edb9817fdd10ff54
• https://git.kernel.org/stable/c/46f52c89a7e7d2691b97a9728e4591d071ca8abc
• https://git.kernel.org/stable/c/774d83b008eccb1c48c14dc5486e7aa255731350
• https://git.kernel.org/stable/c/f126ce7305fe88f49cdabc6db4168b9318898ea3
• https://git.kernel.org/stable/c/b890d45aaf02b564e6cae2d2a590f9649330857d
• https://git.kernel.org/stable/c/47388e807f85948eefc403a8a5fdc5b406a65d5a