Linux Kernel 6.18 RC2 Released: Addressing Regressions, ARM64 KVM Fixes

Linus Torvalds released the second release candidate (RC2) for the Linux Kernel version 6.18. While Linux 6.18-rc2 is larger due to addressing several regressions identified by automated testing, the overall development progress is normal.

6.18 RC1 Regressions

The release candidate 2 brings a large number of important fixes and updates.

The process was not completely smooth. Guenter's automated test system reported several regressions in the previous release, rc1.

Fortunately, some of those problems were just simple setup mistakes, or maybe they were bugs in the QEMU test environment itself.

For example, issues with big-endian SH4 seemed like QEMU problems rather than new kernel errors. However, some regressions are still present, so the community must keep testing.

In the Kernel 6.18-rc2 release announcement mail, Linus Torvalds said they are making slow progress, and he is not worried yet.

What's New in Kernel 6.18-rc2

The Linux 6.18-rc2 release is primarily a fix release that addresses numerous issues, including regressions reported in rc1.

The most significant updates and fixes in 6.18-rc2 are concentrated in virtualization, filesystems (especially SMB client crypto), and graphics drivers.

1. Virtualization (KVM) and ARM64 Architecture

KVM received extensive patches, particularly targeting the ARM64 architecture and guest memory management:

• KVM Guest Memory: Work was done to rework KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD_FLAGS. A new INIT_SHARED flag was added, and SHARED Guest Physical Addresses (GPAs) are now invalidated if the memory supports this flag. This update also enables mmap() on guest_memfd for x86 virtual machines (VMs) using private memory.
• KVM ARM64 Fixes: There were 15 fixes and updates from Marc Zyngier. These include revamping the HCR_EL2 detection logic, fixing WFxT handling related to nested virtualization, and updating timer context helpers.
• Selftests: Multiple KVM selftests were updated or added, specifically for guest_memfd handling and to enable IRQs in vgic_lpi_stress tests.
• Other KVM Fixes: Updates were made to guard PMSCR_EL1 initialization based on the presence of Statistical Profiling Extension (SPE), and to prevent access to vCPU events before initialization.

2. Filesystems and Storage

A major focus was on updating the SMB/CIFS client to use modern cryptographic libraries, alongside crucial stability fixes for BTRFS and EXT4.

• SMB/CIFS Cryptography Overhaul:
  • The SMB client was updated to use the SHA-512 library for the SMB3.1.1 preauth hash.
  • It now utilizes the HMAC-SHA256 library for both key generation and SMB2 signature calculation.
  • The MD5 library is now used for M-F symlink hashing and SMB1 signature calculation, and HMAC-MD5 is used for NTLMv2.
  • Obsolete crypto allocations were removed.
  • Extensive effort also went into improving the smbdirect implementation logic, including managing memory regions (MRs) and ensuring correct memory de-registration.
• BTRFS Fixes: Issues addressed included fixing incorrect readahead expansion length, resolving a memory leak in the qgroup assign ioctl, and ensuring proper handling when rejecting non-SINGLE data profiles.
• EXT4 and JBD2: Fixes were introduced to detect invalid INLINE_DATA + EXTENTS flag combinations in ext4. Updates ensure that all ongoing I/O completes before blocks are freed in both jbd2 and ext4. Orphan information is now freed using kvfree.
• NFS: Updates were merged to apply delay_retrans to asynchronous operations and to fix missing state renewals after boot.

3. Drivers (DRM/Graphics and Networking)

The graphics (DRM) and networking stacks received critical fixes:

• DRM/AMDGPU: Fixes covered handling harvesting for firmware, addressing wrap-around in reemit logic, setting errors on fences from a bad context, and resolving NULL pointer dereferences in VRAM logic for APU devices. Updates were also made to the initialization of the doorbell array.
• DRM/xe: Fixes targeted memory migration issues, ensuring data is migrated to the system if indicated by madvise, correcting color truncation, and increasing the global invalidation timeout.
• Networking/TLS: The Transport Layer Security (TLS) layer received seven fixes, including logic to trim the encrypted message length to match plaintext on short splice, and ensuring waiting for pending asynchronous decryptions or encryptions in error scenarios.

4. Core and Tooling Updates

• BPF: Fixes addressed context leaks in error paths for bpf_prog_test_run_xdp and memory leaks in the __lookup_instance error path.
• x86/MM: A fix was introduced for SMP ordering in switch_mm_irqs_off(), and an overflow issue in __cpa_addr() was resolved.
• Rust Support: The release included cleanups for formatting and addressing a Rust 1.92.0 unused_unsafe warning in the Rust bitmap module.
• Configuration: The long-stale CONFIG_EXT3_FS defconfig option was removed.

Test Linux Kernel 6.18 RC2

Linux 6.18-rc2 brings essential fixes across the board.Please keep testing this release candidate to help developers ensure a smooth final 6.18 kernel launch.

You can download the Kernel 6.18 RC2 from the Kernel.org website or the Linus Torvalds's git tree.

The final stable release of Linux Kernel 6.18 is expected to be around the end of November or the beginning of December 2025.