MIPS: Fix FPU disable with preemption
The FPU should not be left enabled after a task context switch. This isn't usually a problem as the FPU enable bit is updated before returning to userland, however it can potentially mask kernel bugs, and in fact KVM assumes it won't happen and won't clear the FPU enable bit before returning to the guest, which allows the guest to use stale FPU context. Interrupts and exceptions save and restore most bits of the CP0 Status register which contains the FPU enable bit (CU1). When the kernel needs to enable or disable the FPU (for example due to attempted FPU use by userland, or the scheduler being invoked) both the actual Status register and the saved value in the userland context are updated. However this doesn't work correctly with full kernel preemption enabled, since the FPU enable bit can be cleared from within an interrupt when the scheduler is invoked, and only the userland context is updated, not the interrupt context. For example: 1) Enter kernel with FPU already enabled, TIF_USEDFPU=1, Status.CU1=1 saved. 2) Take a timer interrupt while in kernel mode, Status.CU1=1 saved. 3) Timer interrupt invokes scheduler to preempt the task, which clears TIF_USEDFPU, disables the FPU in Status register (Status.CU1=0), and the value stored in user context from step (1), but not the interrupt context from step (2). 4) When the process is scheduled back in again Status.CU1=0. 5) The interrupt context from step (2) is restored, which sets Status.CU1=1. So from user context point of view, preemption has re-enabled FPU! 6) If the scheduler is invoked again (via preemption or voluntarily) before returning to userland, TIF_USEDFPU=0 so the FPU is not disabled before the task context switch. 7) The next task resumes from the context switch with FPU enabled! The restoring of the Status register on return from interrupt/exception is already selective about which bits to restore, leaving the interrupt mask bits alone so enabling/disabling of CPU interrupt lines can persist. Extend this to also leave both the CU1 bit (FPU enable) and the FR bit (which specifies the FPU mode and gets changed with CU1). This prevents a stale Status value being restored in step (5) above and persisting through subsequent context switches. Also switch to the use of definitions from asm/mipsregs.h while we're at it. Since this change also affects the restoration of Status register on the path back to userland, it increases the sensitivity of the kernel to the problem of the FPU being left enabled, allowing it to propagate to userland, therefore a warning is also added to lose_fpu_inatomic() to point out any future reoccurances before they do any damage. 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/12303/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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@ -179,6 +179,10 @@ static inline void lose_fpu_inatomic(int save, struct task_struct *tsk)
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if (save)
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_save_fp(tsk);
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__disable_fpu();
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} else {
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/* FPU should not have been left enabled with no owner */
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WARN(read_c0_status() & ST0_CU1,
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"Orphaned FPU left enabled");
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}
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KSTK_STATUS(tsk) &= ~ST0_CU1;
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clear_tsk_thread_flag(tsk, TIF_USEDFPU);
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@ -289,7 +289,7 @@
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.set reorder
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.set noat
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mfc0 a0, CP0_STATUS
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li v1, 0xff00
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li v1, ST0_CU1 | ST0_IM
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ori a0, STATMASK
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xori a0, STATMASK
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mtc0 a0, CP0_STATUS
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@ -330,7 +330,7 @@
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ori a0, STATMASK
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xori a0, STATMASK
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mtc0 a0, CP0_STATUS
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li v1, 0xff00
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li v1, ST0_CU1 | ST0_FR | ST0_IM
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and a0, v1
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LONG_L v0, PT_STATUS(sp)
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nor v1, $0, v1
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