CVE-2026-98627 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
bpf: Preserve packet pointer class displacement in regsafe()
regsafe() maps packet pointer IDs between states and checks that each current register range is a subset of the corresponding explored register range. It does not, however, preserve the displacement between registers that share a packet pointer ID.
This is unsound because packet range is shared by ID. A bounds check on one class member updates every member, and a later access can consume the range through another member. Commit 022ac0750883 ("bpf: use reg->var_off instead of reg->off for pointers") folded the fixed pointer offset into r64 and removed the old off equality check, so two individually narrower registers can prune even when their displacement has changed. The explored path can then license an out-of-bounds packet access on the pruned path.
Require matching range bases for packet pointers with an ID. Together with the existing ID mapping, this preserves the displacement between members of each packet-pointer class without adding per-ID state. Packet pointers without an ID remain unaffected.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's Berkeley Packet Filter subsystem contains a critical logic flaw within the register safety verification function known as regsafe(). This vulnerability stems from an incomplete implementation of how packet pointer displacements are handled during state exploration and pruning in the eBPF verifier. The core issue lies in the fact that while regsafe() correctly maps packet pointer identifiers between different execution states to ensure consistency, it fails to preserve or verify the specific displacement offsets associated with those pointers when they share a common identifier. In the context of BPF program verification, registers holding packet pointers are grouped into classes based on their shared ID, which implies that bounds checks applied to one member of this class should theoretically apply to all members sharing that same identity. However, because the verifier does not enforce strict equality or subset relationships regarding the actual displacement values within these ranges, it allows for a scenario where two registers with individually narrower valid ranges can be incorrectly pruned from consideration even if their underlying displacements have diverged significantly.
This architectural oversight creates an unsound verification state that directly compromises memory safety guarantees. When the verifier performs bounds checking on one register member of a packet pointer class, it updates the allowed range for that specific ID. Due to the lack of displacement preservation, subsequent accesses utilizing different registers but sharing the same packet pointer ID can consume or expand this pruned range in ways that were not originally validated against the actual memory layout. The root cause was exacerbated by commit 022ac0750883, which refactored how fixed pointer offsets are handled by folding them into a sixty-four-bit variable offset and removing an older equality check mechanism. This optimization inadvertently allowed two registers with distinct but narrow valid ranges to be treated as compatible during pruning operations, effectively licensing out-of-bounds packet accesses on paths that the verifier incorrectly deemed safe. An attacker exploiting this flaw could potentially craft malicious eBPF programs that bypass runtime bounds checks, leading to unauthorized memory reads or writes within kernel space.
The operational impact of this vulnerability is severe, as it undermines one of the primary security assurances provided by the BPF subsystem: isolation between untrusted user-space code and sensitive kernel data structures. By allowing out-of-bounds packet access through manipulated register displacement states, an attacker could potentially leak confidential information from kernel memory or corrupt critical system state. This aligns with CWE-125, Out-of-bounds Read, as the verifier fails to prevent access beyond allocated buffer boundaries due to flawed range analysis logic. Furthermore, this represents a failure in input validation and boundary enforcement within the verification engine itself, which is categorized under CWE-20 Improper Input Validation. From an adversarial perspective, such flaws are often leveraged during post-exploitation phases to escalate privileges or achieve arbitrary code execution by bypassing kernel-level protections, mapping closely to ATT&CK techniques involving memory corruption and privilege escalation via system software vulnerabilities.
To mitigate this risk, the Linux kernel maintainers have implemented a fix that requires matching range bases for packet pointers sharing an identifier. This change ensures that the displacement between members of each packet-pointer class is preserved during verification without introducing additional per-identifier state overhead, thereby maintaining performance while restoring correctness. The solution reinforces the invariant that if two registers share a packet pointer ID, their base ranges must be compatible in terms of both magnitude and offset to prevent false positives in pruning logic. Packet pointers that do not possess an identifier remain unaffected by this change, ensuring backward compatibility with existing programs. System administrators should apply kernel updates containing this patch immediately to close the verification gap. Additionally, developers writing eBPF code should ensure strict adherence to verifier guidelines regarding register usage and avoid relying on implicit assumptions about displacement preservation across different execution paths within shared pointer classes.