# CVE-2026-87039: I2S configure syscall handler passes an unvalidated user-supplied kernel object pointer to drivers

Status: published. Published: 2026-11-04. Updated: 2026-11-04.

The user-mode syscall verifier z_vrfy_i2s_configure() in drivers/i2s/i2s_handlers.c copies the caller's struct i2s_config in from user memory and then, when cfg->frame_clk_freq is 0 (the API's "unconfigure the stream" request), jumps straight to z_impl_i2s_configure() without performing the K_SYSCALL_OBJ(config.mem_slab, K_OBJ_MEM_SLAB) ownership/type check or the block_size sanity check. The mem_slab field is a struct k_mem_slab *, so on that path a completely unvalidated, user-chosen pointer reaches driver code that is entitled to trust it as a real kernel object.

Any thread running under CONFIG_USERSPACE that has access to an I2S device object can trigger this with a single i2s_configure() call. Most in-tree drivers special-case frame_clk_freq == 0 and never store the caller's structure, but drivers/i2s/i2s_litex.c does not: i2s_litex_configure() only logs a warning for a mismatched frame clock, then memcpy()s the caller's config into stream->cfg and marks the stream I2S_STATE_READY. A subsequent i2s_trigger() with I2S_TRIGGER_START enables the FIFO interrupt, and the interrupt handlers i2s_litex_isr_rx() / i2s_litex_isr_tx() then call k_mem_slab_alloc() and k_mem_slab_free() on the attacker-supplied pointer in supervisor context.

The result is an arbitrary-pointer read/write primitive executed by the kernel on behalf of an unprivileged thread — k_mem_slab_alloc() takes a spinlock at slab->lock and reads and writes slab->free_list and the link word inside the returned "block" — which defeats the CONFIG_USERSPACE isolation boundary entirely. The same missing validation is a hazard for out-of-tree I2S drivers that retain the configuration on the unconfigure path. Builds without CONFIG_USERSPACE are unaffected, since the verifier is not used.

The fix zeroes config.mem_slab and config.block_size on the unconfigure path so the driver can never receive an unvalidated pointer, while still allowing a stream to be unconfigured by a caller that does not own a memory slab, as the API documents.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: zephyr) — https://github.com/zephyrproject-rtos/zephyr/commit/e9898c4d22f69575d9b5f297b3df130ceaff1b8f

## Affected products and versions

- zephyrproject / zephyr (zephyr): ≥ 4.4.0; < 4.5.0 (affected, semver). Source: CNA: zephyr

## Severity and prioritization

- CVSS 3.1: 7.8 (high); CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H; source: CNA: zephyr

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-787

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/zephyrproject-rtos/zephyr/commit/e9898c4d22f69575d9b5f297b3df130ceaff1b8f (CNA: zephyr; patch)

- https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-g43m-x6vj-5cg2 (CNA: zephyr)

## Provenance

- CNA: zephyr: https://www.cve.org/CVERecord?id=CVE-2026-87039; retrieved 2026-11-04; source updated 2026-11-04

Revision: 647bb0705279473efe3a8bc97afca6077e96764d0c42d98ca51677053cf89208

Canonical: https://y2.dev/cve/CVE-2026-87039

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