comedi: fix divide-by-zero in comedi_buf_munge()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: comedi: fix divide-by-zero in comedi_buf_munge() The comedi_buf_munge() function performs a modulo operation `async->munge_chan %= async->cmd.chanlist_len` without first checking if chanlist_len is zero. If a user program submits a command with chanlist_len set to zero, this causes a divide-by-zero error when the device processes data in the interrupt handler path. Add a check for zero chanlist_len at the beginning of the function, similar to the existing checks for !map and CMDF_RAWDATA flag. When chanlist_len is zero, update munge_count and return early, indicating the data was handled without munging. This prevents potential kernel panics from malformed user commands.

References

• https://git.kernel.org/stable/c/4ffea48c69cb2b96a281cb7e5e42d706996631db
• https://git.kernel.org/stable/c/8f3e4cd9be4b47246ea73ce5e3e0fa2f57f0d10c
• https://git.kernel.org/stable/c/2670932f2465793fea1ef073e40883e8390fa4d9
• https://git.kernel.org/stable/c/6db19822512396be1a3e1e20c16c97270285ba1a
• https://git.kernel.org/stable/c/d4854eff25efb06d0d84c13e7129bbdba4125f8c
• https://git.kernel.org/stable/c/a4bb5d1bc2f238461bcbe5303eb500466690bb2c
• https://git.kernel.org/stable/c/55520f65fd447e04099a2c44185453c18ea73b7e
• https://git.kernel.org/stable/c/87b318ba81dda2ee7b603f4f6c55e78ec3e95974