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Message-ID: <20180828113611.GC32231@nazgul.tnic>
Date: Tue, 28 Aug 2018 13:36:11 +0200
From: Borislav Petkov <bp@...en8.de>
To: Jann Horn <jannh@...gle.com>
Cc: Kees Cook <keescook@...omium.org>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Andy Lutomirski <luto@...nel.org>,
kernel-hardening@...ts.openwall.com,
Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
dvyukov@...gle.com, linux-edac@...r.kernel.org
Subject: Re: [PATCH v2 1/7] x86: refactor kprobes_fault() like
kprobe_exceptions_notify()
On Mon, Aug 27, 2018 at 08:56:25PM +0200, Jann Horn wrote:
> This is an extension of commit b506a9d08bae ("x86: code clarification patch
> to Kprobes arch code"). As that commit explains, even though
> kprobe_running() can't be called with preemption enabled, you don't have to
> disable preemption - if preemption is on, you can't be in a kprobe.
>
> Also, use X86_TRAP_PF instead of 14.
>
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> arch/x86/mm/fault.c | 19 ++++++++-----------
> 1 file changed, 8 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index b9123c497e0a..2254a30533b9 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -44,17 +44,14 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
>
> static nokprobe_inline int kprobes_fault(struct pt_regs *regs)
> {
> - int ret = 0;
> -
> - /* kprobe_running() needs smp_processor_id() */
> - if (kprobes_built_in() && !user_mode(regs)) {
> - preempt_disable();
> - if (kprobe_running() && kprobe_fault_handler(regs, 14))
> - ret = 1;
> - preempt_enable();
> - }
> -
> - return ret;
> + /*
> + * To be potentially processing a kprobe fault and to be allowed to call
> + * kprobe_running(), we have to be non-preemptible.
> + */
> + if (kprobes_built_in() && !user_mode(regs) && !preemptible() &&
> + kprobe_running() && kprobe_fault_handler(regs, X86_TRAP_PF))
> + return 1;
> + return 0;
Maybe even:
return (kprobes_built_in() &&
!user_mode(regs) &&
!preemptible() &&
kprobe_running() &&
kprobe_fault_handler(regs, X86_TRAP_PF));
Although I'd do it a bit more readable by flipping the checks and
splitting them:
if (!kprobes_built_in())
return 0;
if (user_mode(regs))
return 0;
...
return kprobe_fault_handler(regs, X86_TRAP_PF);
}
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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