linux-kernel - Re: [RFC PATCH 04/10] x86/fpu: eager switch PKRU state

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Date:   Wed, 12 Sep 2018 08:20:48 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [RFC PATCH 04/10] x86/fpu: eager switch PKRU state



> On Sep 12, 2018, at 6:33 AM, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> From: Rik van Riel <riel@...riel.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@...riel.com>
> [bigeasy: load PKRU state only if we also load FPU content]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.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;
> +

I thought that the offset of PKRU in the xstate was fixed after boot.

Anyway, as written, this needs a lockdep assertion that we’re not preemptible, an explicit preempt_disable(), or a comment explaining why it’s okay if we get preempted in this function.

> +    __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)
> {
> -- 
> 2.19.0
>