CVE-2026-74655: serial: qcom-geni: fix TX DMA buffer flush

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

serial: qcom-geni: fix TX DMA buffer flush

When transmit flushing a qcom-geni UART during an ongoing TX DMA, the
UART gets stuck infinitely repeating corrupted TX DMA frames.

The DMA-mode uart_ops does not provide a flush_buffer callback, so an
in-flight transfer can complete after serial core has reset the transmit
kfifo, underflowing its length and resubmitting page-sized transfers
indefinitely. Add one that stops the transfer and clears tx_remaining
and tx_queued.

The stop path was also broken: it unmapped the buffer while the serial
engine could still read it, and never reset the TX DMA state machine.
Cancel the main sequencer command first, then reset the state machine
and wait for it before unmapping. Drop the early return so a pending
mapping is also cleaned up when the main command is inactive.

The bug can be triggered from userspace with a large write immediately
followed by TCOFLUSH. A following tcdrain will hang forever. The bug was
reproduced and this fix was validated on Arduino Uno Q (QRB2210)
using /dev/ttyHS1.

The Linux kernel CVE team has assigned CVE-2026-74655 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.3 with commit 2aaa43c7077833301c237684cd7bc9ae5e3dec95 and fixed in 6.12.104 with commit 313ae287442e5e8d3f7b53b73f37d384bda072d0
	Issue introduced in 6.3 with commit 2aaa43c7077833301c237684cd7bc9ae5e3dec95 and fixed in 6.18.45 with commit 1c31e2377f4c1bb110ca7f6e597b2253a7440c37
	Issue introduced in 6.3 with commit 2aaa43c7077833301c237684cd7bc9ae5e3dec95 and fixed in 7.1.9 with commit b1801c0d40f62778b613334de5840aba10a564d5
	Issue introduced in 6.3 with commit 2aaa43c7077833301c237684cd7bc9ae5e3dec95 and fixed in 7.2 with commit e3c04834ae1ab5e9cfbe8ac54ec734aa4774249d

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74655
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/tty/serial/qcom_geni_serial.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/313ae287442e5e8d3f7b53b73f37d384bda072d0
	https://git.kernel.org/stable/c/1c31e2377f4c1bb110ca7f6e597b2253a7440c37
	https://git.kernel.org/stable/c/b1801c0d40f62778b613334de5840aba10a564d5
	https://git.kernel.org/stable/c/e3c04834ae1ab5e9cfbe8ac54ec734aa4774249d