arm64/fpsimd: Avoid clobbering kernel FPSIMD state with SMSTOP

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: arm64/fpsimd: Avoid clobbering kernel FPSIMD state with SMSTOP On system with SME, a thread's kernel FPSIMD state may be erroneously clobbered during a context switch immediately after that state is restored. Systems without SME are unaffected. If the CPU happens to be in streaming SVE mode before a context switch to a thread with kernel FPSIMD state, fpsimd_thread_switch() will restore the kernel FPSIMD state using fpsimd_load_kernel_state() while the CPU is still in streaming SVE mode. When fpsimd_thread_switch() subsequently calls fpsimd_flush_cpu_state(), this will execute an SMSTOP, causing an exit from streaming SVE mode. The exit from streaming SVE mode will cause the hardware to reset a number of FPSIMD/SVE/SME registers, clobbering the FPSIMD state. Fix this by calling fpsimd_flush_cpu_state() before restoring the kernel FPSIMD state.

References

• https://git.kernel.org/stable/c/55d52af498daea75aa03ba9b7e444c8ae495ac20
• https://git.kernel.org/stable/c/a305821f597ec943849d3e53924adb88c61ed682
• https://git.kernel.org/stable/c/01098d893fa8a6edb2b56e178b798e3e6b674f02