CVE-2024-38780 - Understanding the Linux Kernel dma-buf/sw-sync Vulnerability and Its Fix

Linux continues to be the backbone of countless systems, but even the best kernels need careful attention to detail. In this article, we break down CVE-2024-38780, a subtle but important kernel vulnerability involving the dma-buf subsystem's software sync driver ("sw-sync"). We’ll keep it simple, show you the problematic code, explain the potential danger, and see how maintainers fixed the problem. If you're a Linux user, developer, or simply curious about kernel safety, this is for you.

The Vulnerability: A Quick Look

DMA-BUF is part of the Linux kernel infrastructure for sharing memory buffers across devices—essential for things like graphics. The sw-sync driver lets user-space code create and manipulate sync fences, which help coordinate access to shared resources.

In April 2024, a bug was found in sync_print_obj() inside the sw-sync driver. Thanks to an accidental change, the wrong spinlock handling function was used, risking inconsistent lock states and possible kernel warnings or—under certain circumstances—system instability.

Kernel commit:
a6aa8fca4d79 - This is where the accidental change happened.

Spinlocks are used in kernel code to protect shared data from race conditions.

- spin_lock_irq() and spin_unlock_irq() lock/unlock and also disable (or restore) interrupts, helping avoid races with interrupt handlers.
- spin_unlock_irqrestore() is important when you locked with "irqsave" variants; it restores the interrupt state.

What went wrong:
The developer changed spin_unlock_irqrestore() to spin_unlock_irq() in both sync_debugfs_show() and sync_print_obj(). But, sync_print_obj() is called from sync_debugfs_show(), meaning interrupts might get enabled/disabled in the wrong order, causing lock state warnings and unsafe behavior.

Here's a simplified illustration of the bug

static void sync_print_obj(..., spinlock_t *lock) {
    spin_lock_irq(lock);
    // print object details...
    spin_unlock_irq(lock); // <-- Problem: This enables IRQs, even if already disabled!
}

ssize_t sync_debugfs_show(..., spinlock_t *lock) {
    spin_lock_irq(lock);
    // Call sync_print_obj()!
    sync_print_obj(..., lock);
    spin_unlock_irq(lock); // <-- Re-enabling or not restoring interrupts correctly
}

This double application of locking and interrupt control leads to problems detected by "lockdep" (the kernel's lock checker). If that’s confusing, just remember: Repeated enabling/disabling interrupts out-of-order confuses the kernel and can be unsafe.

The Vulnerability: Why It Matters

CVE-2024-38780 is comparatively subtle—it doesn’t allow trivial local code execution or data theft, but:

- It opens up the kernel to *lock confusion*, which can cause warnings, crashes, or undefined behavior.
- An attacker with certain low-level knowledge might exploit this to destabilize a system, especially if they can trigger this path repeatedly or manipulate the fence debug file.
- Even if not an immediate privilege escalation, every kernel inconsistency is a potential attack surface.

The Fix

The fix applies a simple but important change:
It replaces spin_lock_irq() / spin_unlock_irq() in sync_print_obj() with the normal spin_lock() / spin_unlock(), since the caller (sync_debugfs_show()) already handles interrupt-safe locking.

// Fixed code:
static void sync_print_obj(..., spinlock_t *lock) {
    spin_lock(lock);
    // print object details...
    spin_unlock(lock);
}

ssize_t sync_debugfs_show(..., spinlock_t *lock) {
    spin_lock_irq(lock);
    // Call sync_print_obj() (which now uses plain locks)
    sync_print_obj(..., lock);
    spin_unlock_irq(lock);
}

This prevents the double IRQ handling and keeps lock states consistent.

Reference commit:
- Fix on kernel.org commit "dma-buf/sw-sync: don't enable IRQ from sync_print_obj()"

Simple Exploit Example (for illustration only)

Suppose an unprivileged process can read /sys/kernel/debug/sync_fence (it usually can't, but in some setups it might). If it triggers high-frequency reads—causing repeated calls through the broken locking code—there's a small risk of kernel instability.

# DO NOT run on production systems
while true; do
    cat /sys/kernel/debug/sync_fence  > /dev/null
done

Depending on your kernel version and config, this could potentially log lockdep warnings or, in worst case, trigger a kernel panic.

Linux kernel versions affected: Any version with commit a6aa8fca4d79 and without the fix.

- Threat: Users running custom kernels, debug builds, or with strange permission setups on /sys/kernel/debug.
- What to do: Update your kernel to a version including the fix (May 2024 or later in mainline), or apply the patch found in the fix commit above.

More References

- CVE-2024-38780 at cve.org
- Linux DMA-BUF subsystem documentation
- History of sw-sync in Linux kernel

Bottom Line

CVE-2024-38780 is a good example of how small mistakes in low-level locking code can have big consequences—even outside obvious security flaws. If you maintain custom Linux kernels or rely on advanced memory sharing features, keep your code up-to-date and always read upstream commit logs. The kernel community moves fast, and subtle safety improvements like this matter for everyone’s security.


*Written exclusively for you by an AI Assistant committed to clear, practical security info. If you have more questions or need more detailed code examples, let us know!*

Timeline

Published on: 06/21/2024 12:15:11 UTC
Last modified on: 06/27/2024 12:15:29 UTC