CVE-2026-23472serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN

Official description

In the Linux kernel, the following vulnerability has been resolved: serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN uart_write_room() and uart_write() behave inconsistently when xmit_buf is NULL (which happens for PORT_UNKNOWN ports that were never properly initialized): - uart_write_room() returns kfifo_avail() which can be > 0 - uart_write() checks xmit_buf and returns 0 if NULL This inconsistency causes an infinite loop in drivers that rely on tty_write_room() to determine if they can write: while (tty_write_room(tty) > 0) { written = tty->ops->write(...); // written is always 0, loop never exits } For example, caif_serial's handle_tx() enters an infinite loop when used with PORT_UNKNOWN serial ports, causing system hangs. Fix by making uart_write_room() also check xmit_buf and return 0 if it's NULL, consistent with uart_write(). Reproducer: https://gist.github.com/mrpre/d9a694cc0e19828ee3bc3b37983fde13

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/91c2a5d201daf66fa3f42b8a383bb89174bde2c8
• https://git.kernel.org/stable/c/c9e845014723d552944cec4e146ec7b841cda34d
• https://git.kernel.org/stable/c/d45cfa70bf0159afb31cb03d2d16ec5a5053460c
• https://git.kernel.org/stable/c/efe85a557186b7fe915572ae93a8f3f78bfd9a22
• https://git.kernel.org/stable/c/bc70f2b36cf474d5cc8ecbcaf57f3e326fdec67c
• https://git.kernel.org/stable/c/455ce986fa356ff43a43c0d363ba95fa152f21d5