CVE-2026-23383: bpf, arm64: Force 8-byte alignment for JIT buffer to prevent atomic tearing

In the Linux kernel, the following vulnerability has been resolved:

bpf, arm64: Force 8-byte alignment for JIT buffer to prevent atomic tearing

struct bpfplt contains a u64 target field. Currently, the BPF JIT allocator requests an alignment of 4 bytes (sizeof(u32)) for the JIT buffer.

Because the base address of the JIT buffer can be 4-byte aligned (e.g., ending in 0x4 or 0xc), the relative padding logic in buildplt() fails to ensure that target lands on an 8-byte boundary.

This leads to two issues: 1. UBSAN reports misaligned-access warnings when dereferencing the structure. 2. More critically, target is updated concurrently via WRITEONCE() in bpfarchtextpoke() while the JIT'd code executes ldr. On arm64, 64-bit loads/stores are only guaranteed to be single-copy atomic if they are 64-bit aligned. A misaligned target risks a torn read, causing the JIT to jump to a corrupted address.

Fix this by increasing the allocation alignment requirement to 8 bytes (sizeof(u64)) in bpfjitbinarypackalloc(). This anchors the base of the JIT buffer to an 8-byte boundary, allowing the relative padding math in buildplt() to correctly align the target field.