serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN

Summary

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

Timeline

1. Apr 3, 2026, 3:15 PM UTCCVE published
2. Oct 21, 2026, 12:39 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in c9e845014723d552944cec4e146ec7b841cda34d; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in d45cfa70bf0159afb31cb03d2d16ec5a5053460c; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in efe85a557186b7fe915572ae93a8f3f78bfd9a22; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in bc70f2b36cf474d5cc8ecbcaf57f3e326fdec67c; From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 · fixed in 455ce986fa356ff43a43c0d363ba95fa152f21d5

• Linux Linux

  2.6.12

• KernelLinux

  From 2.6.12 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.114; From 6.13.0 · fixed in 6.18.20; From 6.19.0 · fixed in 6.19.10

References 5