CVE-2026-90023 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
usb: gadget: f_mass_storage: fix null pointer dereference in fsg_common_set_num_buffers()
Previously fsg_num_buffers_validate() was removed as it was not necessary due to Kconfig setting the limits for n from 2 to 256 with default as 2. However, setting the page content in such a way that kstrtou8() reflects n value as either 0 or 1 bypasses these restrictions leading to a null pointer dereference if n is 0. Fix this by adding a check for n < 2 and returning -EINVAL if n is either 0 or 1 consistent with Kconfig logic.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel's USB gadget subsystem, specifically the file-backed mass storage function driver known as f_mass_storage, contained a critical input validation flaw that could lead to a null pointer dereference. This vulnerability arises from an insufficient check on the number of buffers allocated for data transfer operations within the fsg_common_set_num_buffers() function. The underlying issue stems from a previous code change where the dedicated validation routine fsg_num_buffers_validate() was removed under the assumption that kernel configuration options (Kconfig) would enforce safe limits on the variable n, which represents the buffer count. While Kconfig restricts this value to a range of 2 through 256 with a default of 2 during compilation time, it does not prevent runtime manipulation via user-space interfaces or sysfs attributes that allow dynamic configuration changes.
The technical flaw occurs when an attacker or misconfigured application sets the page content in such a way that the kstrtou8() function parses the input value as either zero or one. Since the explicit validation logic was removed, the code proceeds to allocate resources based on this invalid count without verifying its integrity against operational requirements. If n is set to zero, subsequent operations attempt to access memory structures expecting at least two buffers for proper double-buffering and data handling mechanisms inherent in USB mass storage implementations. This mismatch results in a null pointer dereference when the kernel attempts to reference buffer descriptors or queue entries that were never properly initialized due to the skipped allocation steps triggered by an invalid count of zero.
From a security perspective, this vulnerability is classified under CWE-20 Improper Input Validation and CWE-476 NULL Pointer Dereference. The attack vector typically involves local privilege escalation if the user has access to modify USB gadget configurations via sysfs or similar interfaces, although remote exploitation scenarios are less likely given the physical nature of USB device emulation. In terms of offensive security frameworks, this aligns with ATT&CK techniques related to resource manipulation and potential denial of service through kernel panic induction. The impact includes system instability, where a triggered null pointer dereference causes an immediate kernel oops or crash, leading to a denial of service for the host machine running the Linux kernel instance acting as a USB device.
To mitigate this vulnerability, it is essential to restore robust input validation within the fsg_common_set_num_buffers() function. The fix involves explicitly checking if the parsed value n is less than two and returning an error code such as -EINVAL immediately upon detection of invalid buffer counts. This approach ensures that runtime configurations remain consistent with the compile-time constraints enforced by Kconfig, thereby preventing any attempt to allocate zero or insufficient buffers. System administrators should ensure their kernels are updated with patches addressing this specific issue in the usb gadget subsystem. Additionally, defense-in-depth strategies such as enabling kernel hardening features like CONFIG_STRICT_DEVMEM and using SELinux or AppArmor policies can help restrict unauthorized modifications to USB gadget parameters, reducing the attack surface for local users attempting to exploit this flaw.