CVE-2026-74601: ring-buffer: Use current_context for safe per-CPU buffer swap

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

ring-buffer: Use currentcontext for safe per-CPU buffer swap

The ringbufferswapcpu() function currently checks the per-CPU committing counter to determine if a buffer is actively being written to before performing the swap. However, there exists a race window where this check can be bypassed:

ringbufferlockreserve cpubuffer = buffer->buffers[cpu]; // cpubuffera rbreservenextevent rbstartcommit // inc committing if (unlikely(READONCE(cpubuffer->buffer) != buffer)) {...} rbreservenext rbmovetail rbendcommit(cpubuffer); // dec committing => 0 / interrupt hits here, successfully swaps! / localinc(&cpubuffer->committing);

ringbufferunlockcommit cpubuffer = buffer->buffers[cpu]; // cpubufferb rbcommit rbendcommit RBWARNON(cpubuffer, !localread(&cpubuffer->committing)) // triggers warning

The committing counter can temporarily drop to 0 during a single write operation (within rbmovetail), creating a window where swap can succeed even though the write is still in progress. This leads to inconsistent buffer state and triggers the RBWARNON in rbcommit().

Replace the committing counter check with currentcontext checks, which are set at the entry of ringbufferlockreserve() and remain valid throughout the entire write operation, providing a reliable indicator of buffer busy state during swap.