Buffer Overflow in FreeBSD /dev/dsp Kernel Memory Access

Impact Analysis

This vulnerability allows an unprivileged local user to read and write kernel memory, which can be exploited to escalate privileges and potentially gain full control of the affected system.

At a minimum, an attacker can cause the kernel to crash, resulting in a Denial of Service (DoS) condition.

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Executive Summary

The vulnerability exists in the dsp_mmap_single() function, which validates a requested memory mapping by adding a user-supplied offset and length and comparing the sum against the buffer size. However, this addition can overflow, allowing a large offset and length to wrap around and incorrectly pass the validation check.

After this, the offset is narrowed from 64 bits to 32 bits when converted to a buffer address, which results in a mapping that extends beyond the intended audio buffer and into unrelated kernel memory.

Because the /dev/dsp device nodes are world-accessible by default on systems with an audio device, an unprivileged local user can exploit this flaw to read and write kernel memory.

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Mitigation Strategies

The vulnerability involves the /dev/dsp device nodes which are world-accessible by default, allowing unprivileged local users to exploit the flaw. Immediate mitigation steps include restricting access to the /dev/dsp device nodes to trusted users only.

• Change the permissions of /dev/dsp to limit access, for example using: chmod 660 /dev/dsp
• Change the ownership of /dev/dsp to a specific group and add only trusted users to that group, for example: chown root:audio /dev/dsp and usermod -aG audio <trusted_user>
• If possible, disable or unload the audio device driver temporarily until a patch or fix is applied.

These steps help prevent unprivileged users from accessing the vulnerable device and exploiting the kernel memory corruption.

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