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CVE-2026-63883
serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem drivers/tty. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-63883 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
When uart_flush_buffer() runs before the DMA completion IRQ is delivered, the following race can occur (all steps serialized by uart_port_lock): 1. DMA starts: tx_remaining = N, kfifo contains N bytes 2. DMA completes in hardware; IRQ is pending but not yet delivered 3. uart_flush_buffer() acquires the port lock and calls kfifo_reset(), making kfifo_len() = 0 while tx_remaining remains N 4. uart_flush_buffer() releases the port lock 5. DMA IRQ fires; handle_tx_dma() acquires the port lock and calls uart_xmit_advance(uport, tx_remaining) on an empty kfifo uart_xmit_advance() increments kfifo->out by tx_remaining. Since kfifo_reset() already set both in and out to 0, out wraps past in, causing kfifo_len() to return UART_XMIT_SIZE - tx_remaining. The next start_tx_dma() call then submits a DMA transfer of stale buffer data. Fix this by snapshotting kfifo_len() at the start of handle_tx_dma() and skipping uart_xmit_advance() when fifo_len < tx_remaining, which indicates the kfifo was reset by a preceding flush.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
drivers/tty/serial/qcom_geni_serial.c
- What this is
- ARM/QUALCOMM MAILING LISTMAINTAINERS section matched on drivers/*/*/qcom* · linux-arm-msm@vger.kernel.org
Check it on the machine
grep -w CONFIG_SERIAL_QCOM_GENI /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.
- bookwormYnot built on amd64/amd64, amd64/cloud-amd64, amd64/rt/amd64, armhf/armmp, armhf/armmp-lpae, armhf/rt/armmp, i386/686, i386/686-pae, i386/rt/686-pae, ppc64el/powerpc64le
- trixieYnot built on amd64/amd64, amd64/cloud-amd64, amd64/rt/amd64, armel/rpi, armhf/armmp, armhf/armmp-lpae, armhf/rt/armmp, ppc64el/powerpc64le, ppc64el/powerpc64le-64k, riscv64/riscv64, s390x/s390x
- forky, sidYnot built on amd64/amd64, amd64/cloud-amd64, amd64/rt-amd64, armhf/armmp, armhf/armmp-lpae, loong64/loong64, ppc64el/powerpc64le, ppc64el/powerpc64le-64k, riscv64/riscv64, s390x/s390x
- Y
- built in where it is built at all; some flavours do not build it
- Build switch
- CONFIG_SERIAL_QCOM_GENI
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 7.3 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Lsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 5.5% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- drivers/tty
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.3 | 6.6.143 | c91ea13375f7 |
| 6.3 | 6.12.93 | b1159dce10b3 |
| 6.3 | 6.18.35 | 654f45a8569f |
| 6.3 | 7.0.12 | 0d2c41a8b009 |
| 6.3 | 7.1 | 452d6fa37ae9 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The bug is reachable only through local TTY syscalls (open/write/ioctl/close) on Qualcomm GENI high-speed UART device nodes (e.g. /dev/ttyHS*); it is not triggered by remote network packets or physical bus manipulation alone.
- AC:LAttack complexity, low
- An attacker who can open the port controls both sides of the race by writing to start DMA TX and immediately issuing TCFLSH/TCOFLUSH (or closing the port) to call uart_flush_buffer() before the DMA completion IRQ runs, and can repeat until the window is hit.
- PR:LPrivileges required, low
- Exploitation requires only the ability to open and ioctl the UART TTY device (typically dialout-group membership or equivalent service access on Qualcomm Android/automotive/embedded platforms), not init-namespace root or capabilities obtainable only outside user namespaces.
- UI:NUser interaction, none
- Triggering the flush-vs-DMA-IRQ race requires no victim interaction beyond the attacker's own writes and flush/close operations on the serial port.
- S:UScope, unchanged
- Impact is confined to kernel serial-driver state and data transmitted on the local UART; it does not cross VM, container, or IOMMU security boundaries.
- C:HConfidentiality impact, high
- The kfifo index underflow corrupts transmit FIFO bookkeeping and can cause a spurious large DMA read of stale xmit_buf contents (up to UART_XMIT_SIZE/PAGE_SIZE), leaking prior sensitive serial traffic (modem/BT/RS485 diagnostics) that should have been discarded by flush.
- I:HIntegrity impact, high
- The corrupted kfifo state causes unauthorized transmission of stale or attacker-influenced buffer data over the UART/RS485 line, corrupting the integrity of serial output to connected devices (ECUs, modems, factory controllers) beyond what the attacker legitimately queued.
- A:LAvailability impact, low
- The primary impact is corrupted driver transmit state and spurious/erroneous serial output rather than a demonstrated kernel oops/panic; however, repeated triggering can degrade or disrupt serial communication relied on by connected subsystems.