The following commit has been merged into the x86/fpu branch of tip: Commit-ID: 509e7a30cd0a9f38abac4114832d9f69ff0d73b4 Gitweb: https://git.kernel.org/tip/509e7a30cd0a9f38abac4114832d9f69ff0d73b4 Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx> AuthorDate: Fri, 15 Oct 2021 03:16:06 +02:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Wed, 20 Oct 2021 15:27:26 +02:00 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: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Link: https://lkml.kernel.org/r/20211015011538.839822981@xxxxxxxxxxxxx --- arch/x86/kernel/fpu/core.c | 37 ++++++++++++++++++++++++++----------- 1 file changed, 26 insertions(+), 11 deletions(-) diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c index 191269e..9a6b195 100644 --- a/arch/x86/kernel/fpu/core.c +++ b/arch/x86/kernel/fpu/core.c @@ -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; @@ -260,6 +269,23 @@ int fpu_clone(struct task_struct *dst) 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 * child's FPU context, without any memory-to-memory copying. @@ -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. */
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