linux-kernel - Re: [PATCH v3] riscv: Ensure only ASIDLEN is used for sfence.vma

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Message-ID: <CAAhSdy2S26jCmUJES3_63jZbn=0uO78=3kob-AAf0a==4d_seA@mail.gmail.com>
Date:   Thu, 31 Mar 2022 12:08:31 +0530
From:   Anup Patel <anup@...infault.org>
To:     Alistair Francis <alistair.francis@...nsource.wdc.com>
Cc:     Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Atish Patra <atishp@...osinc.com>, Guo Ren <guoren@...nel.org>,
        Alistair Francis <alistair23@...il.com>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH v3] riscv: Ensure only ASIDLEN is used for sfence.vma

On Thu, Mar 31, 2022 at 11:29 AM Alistair Francis
<alistair.francis@...nsource.wdc.com> wrote:
>
> From: Alistair Francis <alistair.francis@....com>
>
> When we set the value of context.id using __new_context() we set both
> the asid and the current_version with this return statement in
> __new_context():
>
>     return asid | ver;
>
> This means that when local_flush_tlb_all_asid() is called with the asid
> specified from context.id we can write the incorrect value.
>
> We get away with this as hardware ignores the extra bits, as the RISC-V
> specification states:
>
> "bits SXLEN-1:ASIDMAX of the value held in rs2 are reserved for future
> standard use. Until their use is defined by a standard extension, they
> should be zeroed by software and ignored by current implementations."
>
> but it is still a bug and worth addressing as we are incorrectly setting
> extra bits.
>
> This patch uses asid_mask when calling sfence.vma to ensure the asid is
> always the correct len (ASIDLEN). This is similar to what we do in
> arch/riscv/mm/context.c.
>
> Fixes: 3f1e782998cd ("riscv: add ASID-based tlbflushing methods")
> Signed-off-by: Alistair Francis <alistair.francis@....com>

Looks good to me.

Reviewed-by: Anup Patel <anup@...infault.org>

Regards,
Anup

> ---
> v3:
>  - Use helper function
> v2:
>  - Pass in pre-masked value
>
>  arch/riscv/include/asm/mmu_context.h | 2 ++
>  arch/riscv/mm/context.c              | 5 +++++
>  arch/riscv/mm/tlbflush.c             | 2 +-
>  3 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
> index 7030837adc1a..94e82c9e17eb 100644
> --- a/arch/riscv/include/asm/mmu_context.h
> +++ b/arch/riscv/include/asm/mmu_context.h
> @@ -16,6 +16,8 @@
>  void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>         struct task_struct *task);
>
> +unsigned long get_mm_asid(struct mm_struct *mm);
> +
>  #define activate_mm activate_mm
>  static inline void activate_mm(struct mm_struct *prev,
>                                struct mm_struct *next)
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 7acbfbd14557..14aec5bacbc1 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -302,6 +302,11 @@ static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu)
>  #endif
>  }
>
> +unsigned long get_mm_asid(struct mm_struct *mm)
> +{
> +       return atomic_long_read(&mm->context.id) & asid_mask;
> +}
> +
>  void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>         struct task_struct *task)
>  {
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 37ed760d007c..9c89c4951bee 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -42,7 +42,7 @@ static void __sbi_tlb_flush_range(struct mm_struct *mm, unsigned long start,
>         /* check if the tlbflush needs to be sent to other CPUs */
>         broadcast = cpumask_any_but(cmask, cpuid) < nr_cpu_ids;
>         if (static_branch_unlikely(&use_asid_allocator)) {
> -               unsigned long asid = atomic_long_read(&mm->context.id);
> +               unsigned long asid = get_mm_asid(mm);
>
>                 if (broadcast) {
>                         sbi_remote_sfence_vma_asid(cmask, start, size, asid);
> --
> 2.35.1
>