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Subject[tip: x86/fpu] x86/fpu: Do not inherit FPU context for kernel and IO worker threads
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@linutronix.de>
AuthorDate: Fri, 15 Oct 2021 03:16:06 +02:00
Committer: Borislav Petkov <bp@suse.de>
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@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.839822981@linutronix.de
---
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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