Overview
CVE-2025-40222 is a vulnerability identified in the Linux kernel’s SH-SCI (SuperH Serial Communication Interface) serial driver. This flaw, if triggered, can lead to a kernel panic due to an out-of-bounds memory access when handling RSCI (Reduced Serial Communication Interface) FIFO overruns. This article provides a detailed technical analysis of the vulnerability, its potential impact, and the necessary mitigation steps to secure your Linux systems.
Technical Details
The vulnerability lies within the error handling code for receive operations in the SH-SCI serial driver. The sci_handle_fifo_overrun() function, responsible for managing FIFO overruns, incorrectly accessed memory outside of the rsci_port_params structure when dealing with RSCI ports. This occurred because the overrun_reg member was interpreted differently for RSCI and other SCIF (Serial Communication Interface with FIFO) port types.
For RSCI, overrun_reg represents a memory offset (CSR at 0x48), whereas for other SCIF types, it’s an enum value used to index into the sci_port_params->regs array. This discrepancy caused the sci_getreg() call to index outside the bounds of the regs array (size 20, defined by SCI_NR_REGS) when processing RSCI overruns.
The out-of-bounds access resulted in reading memory that happened to be interpreted as a plat_sci_reg with a non-zero size. This size mismatch triggered a WARN message within the sci_serial_in() function, as the accidental size didn’t match the supported register sizes. The code snippet provided in the CVE description demonstrates this issue:
Invalid register access
WARNING: CPU: 0 PID: 0 at drivers/tty/serial/sh-sci.c:522 sci_serial_in+0x38/0xac
Modules linked in: renesas_usbhs at24 rzt2h_adc industrialio_adc sha256 cfg80211 bluetooth ecdh_generic ecc rfkill fuse drm backlight ipv6
CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.17.0-rc1+ #30 PREEMPT
Hardware name: Renesas RZ/T2H EVK Board based on r9a09g077m44 (DT)
pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : sci_serial_in+0x38/0xac
lr : sci_serial_in+0x38/0xac
sp : ffff800080003e80
...
CVSS Analysis
As the CVE description indicates a severity of N/A and a CVSS score of N/A, it suggests that the impact of triggering the WARN message alone does not represent a critical or high severity vulnerability. However, the underlying out-of-bounds read has the potential to destabilize the system, so it’s beneficial to apply the provided fix.
Possible Impact
While the CVE entry indicates N/A for severity, the potential impact of this vulnerability should not be ignored. The out-of-bounds memory access could, under certain circumstances:
- Lead to a kernel panic, resulting in system downtime.
- Potentially expose sensitive kernel data, although this is less likely given the specific code path.
- Cause unpredictable system behavior.
Mitigation and Patch Steps
The vulnerability is resolved in the Linux kernel by modifying the RSCI FIFO overrun handling code. The fix ensures that sci_getreg() is not called for port types that don’t use standard register handling. Instead, the code now uses the ops->read_reg() and ops->write_reg() functions to read and write registers for RSCI correctly.
To mitigate this vulnerability, apply the patch referenced in the CVE details to your Linux kernel. Alternatively, upgrade to a kernel version that includes this fix. Specific steps will vary depending on your Linux distribution. Generally, this involves patching and recompiling the kernel, or using the distribution’s package manager to install a patched kernel version.
