CVE-2021-47131 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
net/tls: Fix use-after-free after the TLS device goes down and up
When a netdev with active TLS offload goes down, tls_device_down is called to stop the offload and tear down the TLS context. However, the socket stays alive, and it still points to the TLS context, which is now deallocated. If a netdev goes up, while the connection is still active, and the data flow resumes after a number of TCP retransmissions, it will lead to a use-after-free of the TLS context.
This commit addresses this bug by keeping the context alive until its normal destruction, and implements the necessary fallbacks, so that the connection can resume in software (non-offloaded) kTLS mode.
On the TX side tls_sw_fallback is used to encrypt all packets. The RX side already has all the necessary fallbacks, because receiving non-decrypted packets is supported. The thing needed on the RX side is to block resync requests, which are normally produced after receiving non-decrypted packets.
The necessary synchronization is implemented for a graceful teardown: first the fallbacks are deployed, then the driver resources are released (it used to be possible to have a tls_dev_resync after tls_dev_del).
A new flag called TLS_RX_DEV_DEGRADED is added to indicate the fallback mode. It's used to skip the RX resync logic completely, as it becomes useless, and some objects may be released (for example, resync_async, which is allocated and freed by the driver).
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Analysis
by VulDB Data Team • 02/27/2025
The vulnerability described in CVE-2021-47131 represents a critical use-after-free condition within the Linux kernel's TLS offload implementation that can lead to system instability and potential privilege escalation. This flaw specifically affects the networking subsystem where TLS offload acceleration is utilized to offload cryptographic operations from the CPU to specialized network hardware. The vulnerability arises from improper handling of TLS contexts when network devices transition between down and up states, creating a scenario where freed memory is accessed after the TLS context has been deallocated but the socket continues to reference it. This type of vulnerability falls under CWE-416, which categorizes use-after-free conditions as a serious memory safety issue that can be exploited by attackers to execute arbitrary code or cause denial of service.
The technical flaw manifests when a network device with active TLS offload experiences a state transition from down to up while maintaining active connections. During the device down state, the tls_device_down function is invoked to halt the offload operations and tear down the TLS context, but the socket object remains intact and continues to reference the now-deallocated TLS context structure. When the device subsequently comes back up and data flow resumes after TCP retransmissions, the system attempts to access the freed memory, resulting in undefined behavior and potential system crashes. The vulnerability demonstrates a classic race condition between device state management and connection lifecycle handling within the kernel's networking stack, where synchronization mechanisms are insufficient to prevent concurrent access to freed resources.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation attacks and denial of service conditions across networked systems. Systems utilizing TLS offload acceleration, particularly high-performance servers and network appliances, become vulnerable to exploitation when network devices experience intermittent connectivity issues or undergo maintenance operations that cause device state transitions. The vulnerability affects both the transmission and reception paths of TLS connections, though the implementation addresses the issue differently for each direction. On the transmit side, the system implements software fallback mechanisms using tls_sw_fallback to encrypt packets, while the receive side already supported non-decrypted packet handling but required additional synchronization to prevent resync requests that could trigger the use-after-free condition. This vulnerability affects systems running Linux kernel versions prior to the fix implementation and impacts any application relying on TLS offload features for network security and performance optimization.
The mitigation strategy implemented in the fix addresses the core synchronization issue by ensuring that TLS contexts remain valid until their normal destruction phase rather than being prematurely deallocated during device down transitions. The solution introduces a new flag TLS_RX_DEV_DEGRADED that indicates when fallback mode is active, preventing unnecessary resync operations that could access freed memory structures. The fix implements proper synchronization between fallback deployment and driver resource release, ensuring that fallback mechanisms are established before driver resources are freed, which prevents the possibility of tls_dev_resync operations occurring after tls_dev_del has been called. This approach aligns with ATT&CK technique T1059.001 for command and scripting interpreter, as it addresses a fundamental kernel-level vulnerability that could be exploited to gain elevated privileges through memory corruption. The fix also incorporates proper resource management patterns that prevent the race condition between device state transitions and connection lifecycle management, thereby reducing the attack surface for exploitation and ensuring that system stability is maintained even during network device state changes. The solution maintains backward compatibility while providing a robust mechanism for graceful degradation when hardware offload capabilities are unavailable, allowing connections to continue operating in software mode rather than terminating abruptly.