linux-kernel - Re: [PATCH 2/2] kfence: only handle kernel mode faults

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Message-ID: <CANpmjNOLSuqeTr8YOnvz-V=eudBwraNvm+9V+YxEBUCm=EFwxw@mail.gmail.com>
Date:   Fri, 14 May 2021 12:52:38 +0200
From:   Marco Elver <elver@...gle.com>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] kfence: only handle kernel mode faults

On Fri, 14 May 2021 at 11:22, Sven Schnelle <svens@...ux.ibm.com> wrote:
>
> Signed-off-by: Sven Schnelle <svens@...ux.ibm.com>
> ---
>  mm/kfence/core.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index bc15e3cb71d5..161df492750c 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -813,6 +813,9 @@ bool kfence_handle_page_fault(unsigned long addr, bool is_write, struct pt_regs
>         enum kfence_error_type error_type;
>         unsigned long flags;
>
> +       if (user_mode(regs))
> +               return false;
> +

I don't think it's required on all architectures, correct? If so, I
think this should be part of the arch-specific code, i.e. just do "if
(user_mode(regs) && kfence_handle_page_fault(...))" or similar.
Because otherwise we'll wonder in future why we ever needed this, and
e.g. determine it's useless and remove it again. ;-) Either that, or a
comment. But I'd prefer to just keep it in the arch-specific code if
required, because it seems to be the exception rather than the norm.

Thanks,
-- Marco