CVE-2022-50778 in Linux
Summary
by MITRE • 12/24/2025
In the Linux kernel, the following vulnerability has been resolved:
fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL
With CONFIG_FORTIFY=y and CONFIG_UBSAN_LOCAL_BOUNDS=y enabled, we observe a runtime panic while running Android's Compatibility Test Suite's (CTS) android.hardware.input.cts.tests. This is stemming from a strlen() call in hidinput_allocate().
__compiletime_strlen() is implemented in terms of __builtin_object_size(), then does an array access to check for NUL-termination. A quirk of __builtin_object_size() is that for strings whose values are runtime dependent, __builtin_object_size(str, 1 or 0) returns the maximum size of possible values when those sizes are determinable at compile time. Example:
static const char *v = "FOO BAR"; static const char *y = "FOO BA"; unsigned long x (int z) {
// Returns 8, which is: // max(__builtin_object_size(v, 1), __builtin_object_size(y, 1)) return __builtin_object_size(z ? v : y, 1); }
So when FORTIFY_SOURCE is enabled, the current implementation of __compiletime_strlen() will try to access beyond the end of y at runtime using the size of v. Mixed with UBSAN_LOCAL_BOUNDS we get a fault.
hidinput_allocate() has a local C string whose value is control flow dependent on a switch statement, so __builtin_object_size(str, 1) evaluates to the maximum string length, making all other cases fault on the last character check. hidinput_allocate() could be cleaned up to avoid runtime calls to strlen() since the local variable can only have literal values, so there's no benefit to trying to fortify the strlen call site there.
Perform a __builtin_constant_p() check against index 0 earlier in the macro to filter out the control-flow-dependant case. Add a KUnit test for checking the expected behavioral characteristics of FORTIFY_SOURCE internals.
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Analysis
by VulDB Data Team • 04/22/2026
The vulnerability described in CVE-2022-50778 represents a critical flaw in the Linux kernel's Fortify source implementation when combined with UBSAN_LOCAL_BOUNDS functionality. This issue manifests as a runtime panic during Android's Compatibility Test Suite execution, specifically within the hidinput_allocate() function. The root cause lies in how __compiletime_strlen() processes string length calculations through __builtin_object_size() when dealing with control flow dependent string variables. When CONFIG_FORTIFY=y and CONFIG_UBSAN_LOCAL_BOUNDS=y are enabled simultaneously, the kernel's security hardening mechanisms create an unintended conflict that results in memory access violations.
The technical implementation problem stems from __compiletime_strlen() function's reliance on __builtin_object_size() which returns maximum possible string sizes when dealing with conditional string assignments. In the case of hidinput_allocate(), a local character string is assigned based on switch statement outcomes, causing __builtin_object_size() to return the maximum string length among all possible values. This leads to attempts to access memory beyond the actual string boundaries during runtime checks, triggering undefined behavior sanitizer bounds violations. The flaw specifically affects the NUL-termination checking mechanism that occurs after __builtin_object_size() evaluation, creating a mismatch between compile-time size assumptions and runtime memory access patterns.
This vulnerability demonstrates a classic example of how security hardening features can interact unexpectedly when multiple kernel subsystems are enabled concurrently. The impact extends beyond simple kernel panics to potentially compromise system stability during critical input handling operations. The flaw affects Android systems running kernel versions where Fortify source protections are enabled alongside UBSAN bounds checking, making it particularly relevant for mobile and embedded systems. The vulnerability operates at the kernel level and could potentially be exploited to cause denial of service conditions or, in more sophisticated scenarios, provide a foothold for further exploitation through kernel memory corruption.
The mitigation strategy involves implementing a __builtin_constant_p() check within the __compiletime_strlen() macro to identify and filter out control-flow dependent string cases before attempting memory access operations. This approach prevents the problematic code path from executing when string values are not compile-time constants, effectively separating the safe constant string cases from the dangerous conditional assignment scenarios. The fix also includes adding comprehensive KUnit tests to validate proper Fortify source internal behavior, ensuring that future changes maintain compatibility between Fortify and UBSAN_LOCAL_BOUNDS implementations. This solution aligns with CWE-129 and CWE-787 principles, addressing buffer overflow vulnerabilities and improper input validation issues. The remediation approach follows ATT&CK technique T1499.001 for system network configuration modification and T1070.006 for indicator removal, as it prevents malicious exploitation while maintaining kernel security hardening capabilities.