CVE-2026-63838: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-63838 Overview

CVE-2026-63838 is a Linux kernel vulnerability in the Renesas Sound (rsnd) ASoC driver. The flaw allows a potential out-of-bounds access of the component_dais[] array within the rsnd_dai_of_node() function. The existing boundary check if (i >= RSND_MAX_COMPONENT) does not guarantee that the final valid array element remains zero-initialized. As a result, loop logic that depends on component_dais[RSND_MAX_COMPONENT] being zero can read past the array bounds. The issue was identified by the Linux Verification Center (linuxtesting.org) using the SVACE static analyzer and has been resolved upstream through multiple stable branch commits.

Critical Impact
Out-of-bounds memory access in the kernel audio subsystem can produce undefined behavior, memory corruption, or kernel instability on affected Renesas sound-enabled systems.

Affected Products

Linux kernel — mainline and stable branches containing the Renesas Sound (rsnd) ASoC driver
Distributions shipping unpatched Linux kernels with the sound/soc/sh/rcar driver enabled
Embedded and automotive platforms based on Renesas R-Car SoCs using ASoC audio

Discovery Timeline

2026-07-19 - CVE-2026-63838 published to NVD
2026-07-19 - Last updated in NVD database

Technical Details for CVE-2026-63838

Vulnerability Analysis

The vulnerability resides in the Renesas Sound ASoC driver (sound/soc/sh/rcar/core.c). The static array component_dais[RSND_MAX_COMPONENT] is zero-initialized at declaration and populated dynamically inside rsnd_dai_of_node() as component nodes are enumerated from device tree data. The iteration relies on encountering a zero-valued sentinel entry to terminate processing.

The boundary check if (i >= RSND_MAX_COMPONENT) prevents writes beyond index RSND_MAX_COMPONENT - 1, but does not enforce that the terminal slot at index RSND_MAX_COMPONENT remains zero. When all valid slots are populated, subsequent code paths that scan the array expecting a zero terminator dereference memory past the array end. This is classified as an out-of-bounds read [CWE-125] in the Linux kernel audio subsystem.

Root Cause

The root cause is an implicit reliance on sentinel termination in a fixed-size array without a corresponding guarantee that the sentinel slot exists within bounds. The loop treats component_dais[] as if it were a zero-terminated list of size RSND_MAX_COMPONENT + 1, while the array is only declared with RSND_MAX_COMPONENT elements.

Attack Vector

Exploitation requires that the Renesas Sound driver be loaded and processing a device tree providing enough audio components to exhaust valid slots. The attack vector is local and constrained to systems using Renesas R-Car audio hardware. No remote or unauthenticated exploitation path has been reported, and there is no evidence of exploitation in the wild. The EPSS score is 0.168%, reflecting a low probability of active exploitation. Refer to the upstream commits listed in the Kernel Git Commit Change references for the corrective patch.

Detection Methods for CVE-2026-63838

Indicators of Compromise

Unexpected kernel warnings or KASAN reports referencing rsnd_dai_of_node or component_dais in kernel logs
Audio subsystem initialization failures on Renesas R-Car platforms
Kernel oops or panic traces originating from sound/soc/sh/rcar/core.c

Detection Strategies

Enable Kernel Address Sanitizer (KASAN) on test builds to surface out-of-bounds accesses in the rsnd driver at runtime
Compare the running kernel version and sound/soc/sh/rcar/core.c source against the fixed commits referenced in the Kernel Git Commit Change advisory
Run static analysis tools such as SVACE, Coverity, or Smatch against kernel source trees to identify the boundary check pattern

Monitoring Recommendations

Collect and forward dmesg and journalctl -k output from Renesas R-Car devices to a centralized log platform for kernel fault correlation
Track kernel package versions across fleet inventories to identify hosts still running vulnerable versions
Monitor vendor and distribution security bulletins for backported fixes to long-term stable kernels

How to Mitigate CVE-2026-63838

Immediate Actions Required

Update affected Linux kernels to a version containing one of the upstream fix commits referenced in the NVD entry
Inventory embedded and automotive systems using Renesas R-Car SoCs to identify hosts running the vulnerable rsnd driver
Apply distribution-provided kernel updates as they become available for long-term stable branches

Patch Information

The fix has been merged into mainline and multiple stable branches. Relevant commits include 134c61925e9e, 15e7b2ac2455, 9f1daac27ca2, a62b3e6e4235, and f9e437cddf6c. The patched code ensures the terminal element of component_dais[] remains a valid zero sentinel, preventing the out-of-bounds condition. Review the Kernel Git Commit Change entries for the exact source changes.

Workarounds

Disable the Renesas Sound driver (CONFIG_SND_SOC_RCAR) in kernel configuration on systems that do not require R-Car audio functionality
Restrict local access on affected embedded devices to trusted users until patched kernels are deployed
Rebuild the kernel from source with the upstream fix applied where distribution packages are not yet available

bash

# Verify kernel version and check for the rsnd driver module
uname -r
modinfo snd_soc_rcar 2>/dev/null | grep -E 'filename|version'

# Optional: disable the module on systems that do not need R-Car audio
echo 'blacklist snd_soc_rcar' | sudo tee /etc/modprobe.d/blacklist-rsnd.conf
sudo update-initramfs -u