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SubjectRe: [RFC PATCH 04/10] x86/fpu: eager switch PKRU state
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On 12/09/2018 15:33, Sebastian Andrzej Siewior wrote:
> From: Rik van Riel <riel@surriel.com>
>
> 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 accesses to userspace memory while running
> in kernel mode also need to abide by the memory permissions
> specified in the protection keys.
>
> The pkru info is put in its own cache line in the fpu struct because
> that cache line is accessed anyway at context switch time, and the
> location of the pkru info in the xsave buffer changes depending on
> what other FPU registers are in use if the CPU uses compressed xsave
> state (on by default).
>
> The initial state of pkru is zeroed out automatically by fpstate_init.
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> [bigeasy: load PKRU state only if we also load FPU content]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> arch/x86/include/asm/fpu/internal.h | 11 +++++++++--
> arch/x86/include/asm/fpu/types.h | 10 ++++++++++
> arch/x86/include/asm/pgtable.h | 6 +-----
> arch/x86/mm/pkeys.c | 14 ++++++++++++++
> 4 files changed, 34 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index 16c4077ffc945..57bd1576e033d 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -573,8 +573,15 @@ static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu)
> bool preload = static_cpu_has(X86_FEATURE_FPU) &&
> new_fpu->initialized;
>
> - if (preload)
> - __fpregs_load_activate(new_fpu, cpu);
> + if (!preload)
> + return;
> +
> + __fpregs_load_activate(new_fpu, cpu);
> + /* Protection keys need to be in place right at context switch time. */
> + if (boot_cpu_has(X86_FEATURE_OSPKE)) {
> + if (new_fpu->pkru != __read_pkru())
> + __write_pkru(new_fpu->pkru);
> + }
> }
>
> /*
> diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
> index 202c53918ecfa..6fa58d37938d2 100644
> --- a/arch/x86/include/asm/fpu/types.h
> +++ b/arch/x86/include/asm/fpu/types.h
> @@ -293,6 +293,16 @@ struct fpu {
> */
> unsigned int last_cpu;
>
> + /*
> + * Protection key bits. These also live inside fpu.state.xsave,
> + * but the location varies if the CPU uses the compressed format
> + * for XSAVE(OPT).
> + *
> + * The protection key needs to be switched out immediately at context
> + * switch time, so it is in place for things like copy_to_user.
> + */
> + unsigned int pkru;
> +
> /*
> * @initialized:
> *
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 690c0307afed0..cc36f91011ad7 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -132,11 +132,7 @@ static inline u32 read_pkru(void)
> return 0;
> }
>
> -static inline void write_pkru(u32 pkru)
> -{
> - if (boot_cpu_has(X86_FEATURE_OSPKE))
> - __write_pkru(pkru);
> -}
> +extern void write_pkru(u32 pkru);
>
> static inline int pte_young(pte_t pte)
> {
> diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
> index 6e98e0a7c9231..c7a7b6bd64009 100644
> --- a/arch/x86/mm/pkeys.c
> +++ b/arch/x86/mm/pkeys.c
> @@ -18,6 +18,20 @@
>
> #include <asm/cpufeature.h> /* boot_cpu_has, ... */
> #include <asm/mmu_context.h> /* vma_pkey() */
> +#include <asm/fpu/internal.h>
> +
> +void write_pkru(u32 pkru)
> +{
> + if (!boot_cpu_has(X86_FEATURE_OSPKE))
> + return;
> +
> + current->thread.fpu.pkru = pkru;
> +
> + __fpregs_changes_begin();
> + __fpregs_load_activate(&current->thread.fpu, smp_processor_id());
> + __write_pkru(pkru);
> + __fpregs_changes_end();
> +}
>
> int __execute_only_pkey(struct mm_struct *mm)
> {
>

I think you can go a step further and exclude PKRU state from
copy_kernel_to_fpregs altogether; you just use RDPKRU/WRPKRU. This also
means you don't need to call __fpregs_* functions in write_pkru.

Thanks,

Paolo

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