CVE-2025-39748 in Linux
Summary
by MITRE • 09/11/2025
In the Linux kernel, the following vulnerability has been resolved:
bpf: Forget ranges when refining tnum after JSET
Syzbot reported a kernel warning due to a range invariant violation on the following BPF program.
0: call bpf_get_netns_cookie 1: if r0 == 0 goto <exit> 2: if r0 & Oxffffffff goto <exit>
The issue is on the path where we fall through both jumps.
That path is unreachable at runtime: after insn 1, we know r0 != 0, but with the sign extension on the jset, we would only fallthrough insn 2 if r0 == 0. Unfortunately, is_branch_taken() isn't currently able to figure this out, so the verifier walks all branches. The verifier then refines the register bounds using the second condition and we end up with inconsistent bounds on this unreachable path:
1: if r0 == 0 goto <exit> r0: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0xffffffffffffffff)
2: if r0 & 0xffffffff goto <exit> r0 before reg_bounds_sync: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0)
r0 after reg_bounds_sync: u64=[0x1, 0] var_off=(0, 0)
Improving the range refinement for JSET to cover all cases is tricky. We also don't expect many users to rely on JSET given LLVM doesn't generate those instructions. So instead of improving the range refinement for JSETs, Eduard suggested we forget the ranges whenever we're narrowing tnums after a JSET. This patch implements that approach.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 11/27/2025
The vulnerability CVE-2025-39748 represents a critical flaw in the Linux kernel's BPF (Berkeley Packet Filter) verifier component that arises from improper handling of register bounds refinement during conditional jump operations. This issue manifests when processing specific BPF bytecode sequences involving the JSET instruction, which performs bitwise AND operations with immediate values. The problem occurs in the kernel's verification logic where the system fails to properly track register state transitions, leading to inconsistent bounds calculations that violate fundamental range invariants.
The technical flaw stems from the verifier's inability to correctly identify unreachable execution paths in conditional jump sequences. When analyzing BPF programs containing consecutive conditional jumps where the second jump depends on a bitwise operation, the verifier attempts to refine register bounds using information from subsequent instructions. In the specific case described, the program structure creates a logical contradiction where the second jump condition cannot be satisfied given the constraints established by the first jump. The register r0 is known to be non-zero after the first conditional jump, yet the bitwise AND operation in the second instruction would only allow fall-through if r0 were zero, creating an unreachable execution path.
This inconsistency in register bound tracking leads to a violation of the range invariant that governs how the verifier maintains consistent bounds for register values. The kernel warning occurs during verification when the system attempts to synchronize register bounds after processing the JSET instruction, resulting in contradictory bounds where a register is simultaneously assigned a range that includes values outside its valid domain. The verifier's internal state becomes corrupted, potentially allowing malicious BPF programs to exploit this inconsistency and bypass security checks designed to prevent unauthorized operations.
The operational impact of this vulnerability extends beyond simple verification failures, as it represents a potential pathway for privilege escalation and bypass of kernel security mechanisms. The BPF subsystem is widely used for network filtering, tracing, and security enforcement, making this vulnerability particularly concerning for systems relying heavily on BPF-based security controls. The issue affects kernel versions where the BPF verifier processes conditional jump sequences without proper handling of the interaction between different types of conditional operations. Attackers could potentially craft malicious BPF programs that exploit this verifier inconsistency to execute arbitrary code or circumvent kernel security policies, particularly in environments where BPF programs are loaded with elevated privileges.
The mitigation strategy implemented in this fix addresses the root cause by implementing a defensive approach that forgets previously calculated range information when processing JSET instructions that narrow tnum (type number) values. This approach prevents the verifier from maintaining inconsistent bounds that could lead to security vulnerabilities, even though it may slightly reduce verification efficiency. The solution aligns with established security practices for handling complex conditional logic in kernel verifiers, where defensive programming techniques are preferred over complex logic improvements that might introduce additional vulnerabilities. This fix demonstrates the principle of least privilege in kernel security where verification systems err on the side of caution when handling potentially inconsistent state transitions.
The vulnerability relates to CWE-691, which covers inadequate protection of code against excessive computation, and also connects to ATT&CK techniques involving privilege escalation through kernel exploitation. The issue highlights the complexity of BPF verification and the challenges in maintaining correct state tracking across different instruction types, particularly when dealing with bitwise operations that can create logical contradictions in conditional execution paths. The fix represents a conservative approach to maintaining kernel security boundaries while acknowledging that the underlying complexity of BPF instruction processing makes perfect range refinement difficult to implement correctly. This vulnerability underscores the importance of defensive programming in kernel security components where the consequences of verification errors can extend far beyond simple functional failures.