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Subject[PATCH 14/22] x86/fpu: Eager switch PKRU state
From: Rik van Riel <>

While most of a task's FPU state is only needed in user space, the
protection keys need to be in place immediately after a context switch.

The reason is that any access to userspace memory while running in
kernel mode also need to abide by the memory permissions specified in
the protection keys.

The "eager switch" is a preparation for loading the FPU state on return
to userland. Instead of decoupling PKRU state from xstate I update PKRU
within xstate on write operations by the kernel.

The read/write_pkru() is moved to another header file so it can easily
accessed from pgtable.h and fpu/internal.h.

For user tasks we should always get the PKRU from the xsave area and it
should not change anything because the PKRU value was loaded as part of
FPU restore.
For kernel kernel threads we now will have the default "allow
everything" written. Before this commit the kernel thread would end up
with a random value which it inherited from the previous user task.

Signed-off-by: Rik van Riel <>
[bigeasy: save pkru to xstate, no cache, don't use __raw_xsave_addr()]
Signed-off-by: Sebastian Andrzej Siewior <>
arch/x86/include/asm/fpu/internal.h | 20 ++++++++++++++++++--
arch/x86/include/asm/fpu/xstate.h | 1 +
2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 795a0a2df135e..7191eb9686827 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -559,8 +559,24 @@ switch_fpu_prepare(struct fpu *old_fpu, int cpu)
static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu)
- if (static_cpu_has(X86_FEATURE_FPU))
- __fpregs_load_activate(new_fpu, cpu);
+ struct pkru_state *pk;
+ u32 pkru_val = 0;
+ if (!static_cpu_has(X86_FEATURE_FPU))
+ return;
+ __fpregs_load_activate(new_fpu, cpu);
+ if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
+ return;
+ if (current->mm) {
+ pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
+ WARN_ON_ONCE(!pk);
+ if (pk)
+ pkru_val = pk->pkru;
+ }
+ __write_pkru(pkru_val);

diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index fbe41f808e5d8..4e18a837223ff 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -5,6 +5,7 @@
#include <linux/types.h>
#include <asm/processor.h>
#include <linux/uaccess.h>
+#include <asm/user.h>

/* Bit 63 of XCR0 is reserved for future expansion */
 \ /
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