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

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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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.839822981@linutronix.de
This commit is contained in:
Thomas Gleixner 2021-10-15 03:16:06 +02:00 committed by Borislav Petkov
parent 2d16a1876f
commit 509e7a30cd
1 changed files with 26 additions and 11 deletions
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37
arch/x86/kernel/fpu/core.c
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@ -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.
*/