CVE-2024-35895 in Linux
Summary
by MITRE • 05/19/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf, sockmap: Prevent lock inversion deadlock in map delete elem
syzkaller started using corpuses where a BPF tracing program deletes elements from a sockmap/sockhash map. Because BPF tracing programs can be invoked from any interrupt context, locks taken during a map_delete_elem operation must be hardirq-safe. Otherwise a deadlock due to lock inversion is possible, as reported by lockdep:
CPU0 CPU1 ---- ---- lock(&htab->buckets[i].lock);
local_irq_disable(); lock(&host->lock); lock(&htab->buckets[i].lock);
lock(&host->lock);
Locks in sockmap are hardirq-unsafe by design. We expects elements to be deleted from sockmap/sockhash only in task (normal) context with interrupts enabled, or in softirq context.
Detect when map_delete_elem operation is invoked from a context which is _not_ hardirq-unsafe, that is interrupts are disabled, and bail out with an error.
Note that map updates are not affected by this issue. BPF verifier does not allow updating sockmap/sockhash from a BPF tracing program today.
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Analysis
by VulDB Data Team • 12/30/2024
The vulnerability CVE-2024-35895 addresses a critical lock inversion deadlock condition in the Linux kernel's BPF (Berkeley Packet Filter) subsystem, specifically within sockmap and sockhash implementations. This issue manifests when BPF tracing programs attempt to delete elements from sockmap/sockhash maps, creating a scenario where the kernel's locking mechanism fails due to improper context handling during interrupt processing. The problem was identified through syzkaller's automated testing framework which utilizes corpuses containing BPF tracing programs that exercise map deletion operations. The vulnerability represents a fundamental flaw in the kernel's concurrent access control mechanisms, particularly when dealing with interrupt contexts where certain locking operations cannot safely occur.
The technical root cause stems from the improper handling of lock acquisition sequences in the map_delete_elem operation. When BPF tracing programs execute in interrupt context, they trigger a specific locking pattern that creates a deadlock condition. The lockdep subsystem detected this issue through a classic lock inversion scenario where CPU0 acquires a hash table bucket lock and CPU1 acquires a host lock before attempting to acquire the same bucket lock, creating a circular dependency. The sockmap locks are designed to be hardirq-unsafe, meaning they cannot safely be acquired when interrupts are disabled, which typically occurs in interrupt contexts. This design constraint conflicts with the BPF tracing program's execution environment, where interrupts may be disabled during map deletion operations.
The operational impact of this vulnerability is significant for systems running BPF programs that interact with sockmap/sockhash functionality, particularly in network monitoring, traffic analysis, and security auditing applications. When triggered, the deadlock condition results in system hangs or kernel panics, effectively rendering the affected kernel instances unresponsive. The vulnerability affects systems where BPF tracing programs are actively deleting elements from sockmap/sockhash maps, which can occur in various network security tools, performance monitoring applications, and kernel-based network filtering solutions. This issue particularly impacts environments using BPF for real-time network packet processing where tracing programs operate in interrupt contexts.
The kernel's resolution implements a context detection mechanism that identifies when map_delete_elem operations occur in interrupt contexts where interrupts are disabled. The fix specifically checks if the operation is invoked from a context that is not hardirq-unsafe, meaning interrupts are disabled, and returns an error instead of proceeding with the potentially deadlock-inducing lock operations. This approach follows established security practices by failing gracefully rather than allowing system instability. The solution aligns with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and addresses ATT&CK techniques related to system compromise through kernel-level vulnerabilities. It's important to note that map update operations remain unaffected by this issue, as the BPF verifier already prevents updating sockmap/sockhash from BPF tracing programs, making this a targeted fix for the deletion operation specifically.