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SubjectRe: [RFC PATCH 04/10] x86/fpu: eager switch PKRU state
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> On Sep 12, 2018, at 7:18 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>> 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.
>
>

Except that the signal ABI has PKRU in the xstate. So we’d need to fake it or do something special for signals.
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