CVE-2026-93125 in Linux
Summary
by MITRE • 09/18/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Reject rdonly/rdwr_buf_size kfunc arguments that exceed u32 max
check_kfunc_args() detects a kfunc argument named rdonly_buf_size or rdwr_buf_size and stores reg->var_off.value into meta->r0_size, a u64, and does not bound it. check_kfunc_call() later copies that value into the returned register's mem_size field:
meta->r0_size = reg->var_off.value; ... regs[BPF_REG_0].mem_size = meta.r0_size;
regs[BPF_REG_0].mem_size is u32. A constant whose upper 32 bits are set
gets truncated instead of causing a load-time rejection, so the verifier records a PTR_TO_MEM register with an approximately 4 GiB mem_size for whatever allocation the kfunc returned. A later access check against that register uses the truncated, wrong bound.
Reject rdonly_buf_size/rdwr_buf_size values that exceed U32_MAX at the point meta->r0_size is set.
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Analysis
by VulDB Data Team • 09/18/2026
The Linux kernel's eBPF verifier contains a critical type mismatch vulnerability within its kfunc argument checking logic, specifically involving the handling of buffer size parameters for read-only and read-write buffers. The function check_kfunc_args identifies arguments named rdonly_buf_size or rdwr_buf_size and assigns their values to meta->r0_size, which is defined as an unsigned 64-bit integer. This assignment occurs without any validation against the maximum value supported by a 32-bit unsigned integer. Subsequently, during the verification of kfunc calls in check_kfunc_call(), this 64-bit value is copied into regs[BPF_REG_0].mem_size, which is strictly an unsigned 32-bit field. Because there is no intermediate bounds checking to ensure the source value fits within the destination type's capacity, any constant argument with its upper thirty-two bits set will undergo silent truncation rather than triggering a load-time rejection by the verifier.
This architectural flaw results in the eBPF program being accepted despite containing an invalid memory size specification that is approximately four gigabytes smaller than intended due to overflow behavior. When the runtime subsequently performs access checks against registers of type PTR_TO_MEM, it relies on this truncated mem_size value as the upper bound for memory operations. Consequently, a verifier that should have rejected an out-of-bounds allocation or pointer arithmetic now permits accesses that exceed the actual allocated buffer size but fall within the artificially reduced thirty-two-bit limit. This discrepancy creates a scenario where malicious or buggy eBPF programs can perform unauthorized reads or writes to kernel memory regions outside their designated boundaries, effectively bypassing the safety guarantees provided by the BPF verifier subsystem.
The operational impact of this vulnerability is severe, as it undermines the fundamental isolation model that protects the Linux kernel from untrusted user-space code executed via eBPF. An attacker could exploit this truncation issue to craft a malicious eBPF program that reads sensitive kernel data or overwrites critical kernel structures by leveraging the incorrect memory bounds. This aligns with CWE-190, Integer Overflow or Wraparound, and specifically relates to improper type conversion leading to security bypasses. In terms of attack vectors, this vulnerability facilitates privilege escalation if exploited in a context where eBPF programs have elevated privileges, such as CAP_BPF or SYS_ADMIN capabilities, allowing the attacker to compromise kernel integrity and potentially achieve full system control.
To mitigate this risk, it is imperative that developers enforce strict type checking at the point of assignment within check_kfunc_args. The logic must explicitly reject any rdonly_buf_size or rdwr_buf_size argument whose value exceeds U32_MAX before storing it in meta->r0_size. This ensures that only valid thirty-two-bit values are propagated to the mem_size field, preventing truncation and maintaining accurate memory bounds for subsequent access checks. System administrators should ensure their kernels are updated with patches addressing this specific verifier logic error, as relying on runtime behavior is insufficient given the deterministic nature of eBPF compilation and verification processes.