CVE-2025-39728 - How a Tiny Linux Kernel Bug Could Crash Your Samsung Device

The Linux kernel is the heart of almost every Android and many server devices, making security and stability fixes crucial to billions of users worldwide. Recently, a kernel bug — now tracked as CVE-2025-39728 — was fixed in the samsung_clk_init() function, preventing systems from crashing during boot due to a subtle error when certain compile-time options were enabled.

This long-form post breaks down the bug, its impact, the patch, and offers a code snippet illustrating what went wrong. Let's explore how this small fix prevents catastrophic panics on affected hardware.

What Is CVE-2025-39728?

CVE-2025-39728 refers to a vulnerability in the Linux kernel’s Samsung clock ("clk") driver. The function samsung_clk_init() was indexing into an array before properly setting its size, leading to a kernel crash under particular debug builds or options.

This is particularly relevant when compiled with UBSAN_ARRAY_BOUNDS (Undefined Behavior Sanitizer), which checks for out-of-bounds array accesses at runtime.

Who Is Affected?

- Devices using the Samsung clock controller kernel code (think Samsung Exynos-based phones, tablets, developer boards, and some ARM servers).
- Kernel builds with the sanitizer flag UBSAN_ARRAY_BOUNDS=y enabled — this is mainly developers, testers, or custom ROM communities, but even end-users could be at risk if this bug is triggered.

Here’s what the kernel log showed

UBSAN: array index out of bounds: 00000000f2005512 [#1] PREEMPT SMP
<snip>
Call trace:
 samsung_clk_init+x110/x124 (P)
 samsung_clk_init+x48/x124 (L)
 samsung_cmu_register_one+x3c/xa
 exynos_arm64_register_cmu+x54/x64
 __gs101_cmu_top_of_clk_init_declare+x28/x60
 ...

Translation:
In samsung_clk_init(), code tried to access ctx->clk_data.hws (an array pointer) before ctx->clk_data.num (the number of array elements) was set. If an out-of-bounds access happened, the sanitizer panics and the kernel crashes.

Here's a simplified (and slightly cleaned up) snippet illustrating the problem

static int samsung_clk_init(..., struct samsung_clk_provider *ctx, ...)
{
    /* ... */

    // BAD: Access array before number-of-clocks is set
    for (i = ; i < nr_clks; i++) {
        ctx->clk_data.hws[i] = ...; // May crash if hws is not sized correctly yet
    }
    ctx->clk_data.num = nr_clks; // Setting num *after* use

    /* ... */
}

The Fix

The fix is simple but crucial:
Assign num (the number of clocks) _before_ using the array.

Patch in pseudo-diff

static int samsung_clk_init(...)
{
-    for (i = ; i < nr_clks; i++) {
-        ctx->clk_data.hws[i] = ...;
-    }
-    ctx->clk_data.num = nr_clks; // too late
+    ctx->clk_data.num = nr_clks; // set this first!
+    for (i = ; i < nr_clks; i++) {
+        ctx->clk_data.hws[i] = ...;
+    }
}

See the official patch on the Linux Kernel Mailing List:
- linux-samsung-clk: Fix UBSAN panic in samsung_clk_init()

Exploit Details

Although this bug is not a classic security exploit (no privilege escalation or direct data leak), it has two important consequences:

Unexpected Behavior:

In fuzzing or sanitizer-enabled environments, such a bug might be used along with others for exploitation, especially if out-of-bounds memory reads or writes are triggered.

Watch the kernel panic during samsung_clk_init().

NOTE:
This is more a demonstration for kernel developers, debugging, and testers with recovery mechanisms ready.

Order matters, even in driver code: Always assign structure fields _before_ using them.

- Sanitizers = safety net: UBSAN and similar tools catch subtle bugs at build/run time, often before devices hit mainline or user hands.
- Keep systems updated: Even corner-case bugs like these can cause critical issues if unpatched, especially in mass-production environments.

References

- linux-samsung-clk: Fix UBSAN panic in samsung_clk_init() Original Patch
- UBSAN in the Linux Kernel Docs
- Linux Kernel source (clk/samsung/clk.c)
- CVE-2025-39728 at cve.org (Pending)

Final Thoughts

CVE-2025-39728 is a good reminder that even minor programming mistakes in low-level, critical code can bring down entire systems. The fix is small — just a line reordered — but the impact is big for device stability.

If you maintain Linux on ARM, Samsung Exynos, or use custom kernels: apply this patch, enable sanitizers, and stay safe!


Stay tuned for more simple-language breakdowns of recent kernel CVEs. If you have any questions or want deeper technical examples, let me know in the comments!

Timeline

Published on: 04/18/2025 07:15:44 UTC
Last modified on: 04/29/2025 14:18:06 UTC