MIPS: Properly disable FPU in start_thread()

start_thread() (called for execve(2)) clears the TIF_USEDFPU flag
without atomically disabling the FPU. With a preemptive kernel, an
unfortunately timed preemption after this could result in another
task (or KVM guest) being scheduled in with the FPU still enabled, since
lose_fpu_inatomic() only turns it off if TIF_USEDFPU is set.

Use lose_fpu(0) instead of the separate FPU / MSA management, which
should do the right thing (drop FPU properly and atomically without
saving state) and will be more future proof.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12302/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index f2975d4d1e449cc948d84dacac736451a3144cf9..eddd5fd6fdfa2ee20f50886b8fbbad9045fd496d 100644 (file)
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -65,12 +65,10 @@ void start_thread(struct pt_regs * regs, unsigned long pc, unsigned long sp)
        status = regs->cp0_status & ~(ST0_CU0|ST0_CU1|ST0_FR|KU_MASK);
        status |= KU_USER;
        regs->cp0_status = status;
+       lose_fpu(0);
+       clear_thread_flag(TIF_MSA_CTX_LIVE);
        clear_used_math();
-       clear_fpu_owner();
        init_dsp();
-       clear_thread_flag(TIF_USEDMSA);
-       clear_thread_flag(TIF_MSA_CTX_LIVE);
-       disable_msa();
        regs->cp0_epc = pc;
        regs->regs[29] = sp;
 }