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What’s new in security for Ubuntu 24.04 LTS?

We’re excited about the upcoming Ubuntu 24.04 LTS release, Noble Numbat. Like all Ubuntu releases, Ubuntu 24.04 LTS comes with 5 years of free security maintenance for the main repository. Support can be expanded for an extra 5 years, and to include the universe repository, via Ubuntu Pro.  Organisations looking to keep their systems secure without needing a major upgrade can also get the Legacy Support add-on to expand that support beyond the 10 years. Combined with the enhanced security coverage provided by Ubuntu Pro and Legacy Support, Ubuntu 24.04 LTS provides a secure foundation on which to develop and deploy your applications and services in an increasingly risky environment. In this blog post, we will look at some of the enhancements and security features included in Noble Numbat, building on those available in Ubuntu 22.04 LTS.

Unprivileged user namespace restrictions

Unprivileged user namespaces are a widely used feature of the Linux kernel, providing additional security isolation for applications, and are often employed as part of a sandbox environment. They allow an application to gain additional permissions within a constrained environment, so that a more trusted part of an application can then use these additional permissions to create a more constrained sandbox environment within which less trusted parts can then be executed. A common use case is the sandboxing employed by modern web browsers, where the (trusted) application itself sets up the sandbox where it executes the untrusted web content. However, by providing these additional permissions, unprivileged user namespaces also expose additional attack surfaces within the Linux kernel. There has been a long history of (ab)use of unprivileged user namespaces to exploit various kernel vulnerabilities. The most recent interim release of Ubuntu, 23.10, introduced the ability to restrict the use of unprivileged user namespaces to only those applications which legitimately require such access. In Ubuntu 24.04 LTS, this feature has both been improved to cover additional applications both within Ubuntu and from third parties, and to allow better default semantics of the feature. For Ubuntu 24.04 LTS, the use of unprivileged user namespaces is then allowed for all applications but access to any additional permissions within the namespace are denied. This allows more applications to more better gracefully handle this default restriction whilst still protecting against the abuse of user namespaces to gain access to additional attack surfaces within the Linux kernel.

Binary hardening

Modern toolchains and compilers have gained many enhancements to be able to create binaries that include various defensive mechanisms. These include the ability to detect and avoid various possible buffer overflow conditions as well as the ability to take advantage of modern processor features like branch protection for additional defence against code reuse attacks.

The GNU C library, used as the cornerstone of many applications on Ubuntu, provides runtime detection of, and protection against, certain types of buffer overflow cases, as well as certain dangerous string handling operations via the use of the _FORTIFY_SOURCE macro. FORTIFY_SOURCE can be specified at various levels providing increasing security features, ranging from 0 to 3. Modern Ubuntu releases have all used FORTIFY_SOURCE=2 which provided a solid foundation by including checks on string handling functions like sprintf(), strcpy() and others to detect possible buffer overflows, as well as format-string vulnerabilities via the %n format specifier in various cases. Ubuntu 24.04 LTS enables additional security features by increasing this to FORTIFY_SOURCE=3. Level three greatly enhances the detection of possible dangerous use of a number of other common memory management functions including memmove(),  memcpy(), snprintf(), vsnprintf(), strtok() and strncat(). This feature is enabled by default in the gcc compiler within Ubuntu 24.04 LTS, so that all packages in the Ubuntu archive which are compiled with gcc, or any applications compiled with gcc on Ubuntu 24.04 LTS also receive this additional protection.

The Armv8-M hardware architecture (provided by the “arm64” software architecture on Ubuntu) provides hardware-enforced pointer authentication and branch target identification. Pointer authentication provides the ability to detect malicious stack buffer modifications which aim to redirect pointers stored on the stack to attacker controlled locations, whilst branch target identification is used to track certain indirect branch instructions and the possible locations which they can target. By tracking such valid locations, the processor can detect possible malicious jump-oriented programming attacks which aim to use existing indirect branches to jump to other gadgets within the code. The gcc compiler supports these features via the -mbranch-protection option. In Ubuntu 24.04 LTS, the dpkg package now enables -mbranch-protection=standard, so that all packages within the Ubuntu archive enable support for these hardware features where available.

AppArmor 4

The aforementioned unprivileged user namespace restrictions are all backed by the AppArmor mandatory access control system. AppArmor allows a system administrator to implement the principle of least authority by defining which resources an application should be granted access to and denying all others. AppArmor consists of a userspace package, which is used to define the security profiles for applications and the system, as well as the AppArmor Linux Security Module within the Linux kernel which provides enforcement of the policies. Ubuntu 24.04 LTS includes the latest AppArmor 4.0 release, providing support for many new features, such as specifying allowed network addresses and ports within the security policy (rather than just high level protocols) or various conditionals to allow more complex policy to be expressed. An exciting new development provided by AppArmor 4 in Ubuntu 24.04 LTS is the ability to defer access control decisions to a trusted userspace program. This allows for quite advanced decision making to be implemented, by taking into account the greater context available within userspace or to even interact with the user / system administrator in a real-time fashion. For example, the experimental snapd prompting feature takes advantage of this work to allow users to exercise direct control over which files a snap can access within their home directory. Finally, within the kernel, AppArmor has gained the ability to mediate access to user namespaces as well as the io_uring subsystem, both of which have historically provided additional kernel attack surfaces to malicious applications. 

Disabling of old TLS versions

The use of cryptography for private communications is the backbone of the modern internet. The Transport Layer Security protocol has provided confidentiality and integrity to internet communications since it was first standardised in 1999 with TLS 1.0. This protocol has undergone various revisions since that time to introduce additional security features and avoid various security issues inherent in the earlier versions of this standard. Given the wide range of TLS versions and options supported by each, modern internet systems will use a process of auto-negotiation to select an appropriate combination of protocol version and parameters when establishing a secure communications link. In Ubuntu 24.04 LTS, TLS 1.0, 1.1 and DTLS 1.0 are all forcefully disabled (for any applications that use the underlying openssl or gnutls libraries) to ensure that users are not exposed to possible TLS downgrade attacks which could expose their sensitive information.

Upstream Kernel Security Features

Linux kernel v5.15 was used as the basis for the Linux kernel in the previous Ubuntu 22.04 LTS release. This provided a number of kernel security features including core scheduling, kernel stack randomisation and unprivileged BPF restrictions to name a few. Since that time, the upstream Linux kernel community has been busy adding additional kernel security features. Ubuntu 24.04 LTS includes the v6.8 Linux kernel which provides the following additional security features:

Intel shadow stack support

Modern Intel CPUs support an additional hardware feature aimed at preventing certain types of return-oriented programming (ROP) and other attacks that target the malicious corruption of the call stack. A shadow stack is a hardware enforced copy of the stack return address that cannot be directly modified by the CPU. When the processor returns from a function call, the return address from the stack is compared against the value from the shadow stack – if the two differ, the process is terminated to prevent a possible ROP attack. Whilst compiler support for this feature has been enabled for userspace packages since Ubuntu 19.10, it has not been able to be utilised until it was also supported by the kernel and the C library. Ubuntu 24.04 LTS includes this additional support for shadow stacks to allow this feature to be enabled when desired by setting the GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK environment variable.

Secure virtualisation with AMD SEV-SNP and Intel TDX

Confidential computing represents a fundamental departure from the traditional threat model, where vulnerabilities in the complex codebase of privileged system software like the operating system, hypervisor, and firmware pose ongoing risks to the confidentiality and integrity of both code and data. Likewise, unauthorised access by a malicious cloud administrator could jeopardise the security of your virtual machine (VM) and its environment. Building on the innovation of Trusted Execution Environments at the silicon level, Ubuntu Confidential VMs aim to restore your control over the security assurances of your VMs.

For the x86 architecture, both AMD and Intel processors provide hardware features (named AMD SEV SNP and Intel TDX respectively) to support running virtual machines with memory encryption and integrity protection. They ensure that the data contained within the virtual machine is inaccessible to the hypervisor and hence the infrastructure operator.  Support for using these features as a guest virtual machine was introduced in the upstream Linux kernel version 5.19.

Thanks to Ubuntu Confidential VMs, a user can make use of compute resources provided by a third party whilst maintaining the integrity and confidentiality of their data through the use of memory encryption and other features.  On the public cloud, Ubuntu offers the widest portfolio of confidential VMs. These build on the innovation of both the hardware features, with offerings available across Microsoft Azure, Google Cloud and Amazon AWS. 

For enterprise customers seeking to harness confidential computing within their private data centres, a fully enabled software stack is essential. This stack encompasses both the guest side (kernel and OVMF) and the host side (kernel-KVM, QEMU, and Libvirt). Currently, the host-side patches are not yet upstream. To address this, Canonical and Intel have forged a strategic collaboration to empower Ubuntu customers with an Intel-optimised TDX Ubuntu build. This offering includes all necessary guest and host patches, even those not yet merged upstream, starting with Ubuntu 23.10 and extending into 24.04 and beyond. The complete TDX software stack is accessible through this github repository. 

This collaborative effort enables our customers to promptly leverage the security assurances of Intel TDX. It also serves to narrow the gap between silicon innovation and software readiness, a gap that grows as Intel continues to push the boundaries of hardware innovation with 5th Gen Intel Xeon scalable processors and beyond.

Strict compile-time bounds checking

Similar to hardening of binaries within the libraries and applications distributed in Ubuntu, the Linux kernel itself gained enhanced support for detecting possible buffer overflows at compile time via improved bounds checking of the memcpy() family of functions. Within the kernel, the FORTIFY_SOURCE macro enables various checks in memory management functions like memcpy() and memset() by checking that the size of the destination object is large enough to hold the specified amount of memory, and if not will abort the compilation process. This helps to catch various trivial memory management issues, but previously was not able to properly handle more complex cases such as when an object was embedded within a larger object. This is quite a common pattern within the kernel, and so the changes introduced in the upstream 5.18 kernel version to enumerate and fix various such cases greatly improves this feature. Now the compiler is able to detect and enforce stricter checks when performing memory operations on sub-objects to ensure that other object members are not inadvertently overwritten, avoiding an entire class of possible buffer overflow vulnerabilities within the kernel.

Wrapping up

Overall, the vast range of security improvements that have gone into Ubuntu 24.04 LTS greatly improve on the strong foundation provided by previous Ubuntu releases, making it the most secure release to date. Additional features within both the kernel, userspace and across the distribution as a whole combine to address entire vulnerability classes and attack surfaces. With up to 12 years of support, Ubuntu 24.04 LTS provides the best and most secure foundation to develop and deploy Linux services and applications. Expanded Security Maintenance, kernel livepatching and additional services are all provided to Ubuntu Pro subscribers to enhance the security of their Ubuntu deployments.

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