Commit 509e7a30 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Borislav Petkov
Browse files

x86/fpu: Do not inherit FPU context for kernel and IO worker threads 



There is no reason why kernel and IO worker threads need a full clone of
the parent's FPU state. Both are kernel threads which are not supposed to
use FPU. So copying a large state or doing XSAVE() is pointless. Just clean
out the minimally required state for those tasks.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.839822981@linutronix.de
parent 2d16a187

arch/x86/kernel/fpu/core.c

+26 −11
Original line number Diff line number Diff line
@@ -212,6 +212,15 @@ static inline void fpstate_init_xstate(struct xregs_state *xsave) 
	xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT | xfeatures_mask_all;

}



static inline unsigned int init_fpstate_copy_size(void)

{

	if (!use_xsave())

		return fpu_kernel_xstate_size;



	/* XSAVE(S) just needs the legacy and the xstate header part */

	return sizeof(init_fpstate.xsave);

}



static inline void fpstate_init_fxstate(struct fxregs_state *fx)

{

	fx->cwd = 0x37f;
@@ -259,6 +268,23 @@ int fpu_clone(struct task_struct *dst) 
	if (!cpu_feature_enabled(X86_FEATURE_FPU))

		return 0;



	/*

	 * Enforce reload for user space tasks and prevent kernel threads

	 * from trying to save the FPU registers on context switch.

	 */

	set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);



	/*

	 * No FPU state inheritance for kernel threads and IO

	 * worker threads.

	 */

	if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {

		/* Clear out the minimal state */

		memcpy(&dst_fpu->state, &init_fpstate,

		       init_fpstate_copy_size());

		return 0;

	}



	/*

	 * If the FPU registers are not owned by current just memcpy() the

	 * state.  Otherwise save the FPU registers directly into the
@@ -272,8 +298,6 @@ int fpu_clone(struct task_struct *dst) 
		save_fpregs_to_fpstate(dst_fpu);

	fpregs_unlock();



	set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);



	trace_x86_fpu_copy_src(src_fpu);

	trace_x86_fpu_copy_dst(dst_fpu);
@@ -322,15 +346,6 @@ static inline void restore_fpregs_from_init_fpstate(u64 features_mask) 
	pkru_write_default();

}



static inline unsigned int init_fpstate_copy_size(void)

{

	if (!use_xsave())

		return fpu_kernel_xstate_size;



	/* XSAVE(S) just needs the legacy and the xstate header part */

	return sizeof(init_fpstate.xsave);

}



/*

 * Reset current->fpu memory state to the init values.

 */