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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Peter Zijlstra <peterz@infradead.org>,
	tglx@linutronix.de, luto@amacapital.net
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	Lai Jiangshan <laijs@linux.alibaba.com>,
	sean.j.christopherson@intel.com, daniel.thompson@linaro.org,
	a.darwish@linutronix.de, rostedt@goodmis.org,
	bigeasy@linutronix.de
Subject: Re: [PATCH 02/14] x86/hw_breakpoint: Prevent data breakpoints on direct GDT
Date: Sat, 30 May 2020 13:45:43 +0100
Message-ID: <582d9136-8f8b-fa07-862e-9ea5d440c09f@citrix.com> (raw)
In-Reply-To: <20200529213320.840953950@infradead.org>

On 29/05/2020 22:27, Peter Zijlstra wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> A data breakpoint on the GDT is terrifying and should be avoided.
> The GDT on CPU entry area is already protected. The direct GDT
> should be also protected, although it is seldom used and only
> used for short time.

While I agree with the sentiment...

>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20200526014221.2119-3-laijs@linux.alibaba.com
> ---
>  arch/x86/kernel/hw_breakpoint.c |   30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
>
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -32,6 +32,7 @@
>  #include <asm/processor.h>
>  #include <asm/debugreg.h>
>  #include <asm/user.h>
> +#include <asm/desc.h>
>  
>  /* Per cpu debug control register value */
>  DEFINE_PER_CPU(unsigned long, cpu_dr7);
> @@ -237,13 +238,26 @@ static inline bool within_area(unsigned
>  }
>  
>  /*
> - * Checks whether the range from addr to end, inclusive, overlaps the CPU
> - * entry area range.
> + * Checks whether the range from addr to end, inclusive, overlaps the fixed
> + * mapped CPU entry area range or other ranges used for CPU entry.
>   */
> -static inline bool within_cpu_entry_area(unsigned long addr, unsigned long end)
> +static inline bool within_cpu_entry(unsigned long addr, unsigned long end)
>  {
> -	return within_area(addr, end, CPU_ENTRY_AREA_BASE,
> -			   CPU_ENTRY_AREA_TOTAL_SIZE);
> +	int cpu;
> +
> +	/* CPU entry erea is always used for CPU entry */
> +	if (within_area(addr, end, CPU_ENTRY_AREA_BASE,
> +			CPU_ENTRY_AREA_TOTAL_SIZE))
> +		return true;
> +
> +	for_each_possible_cpu(cpu) {
> +		/* The original rw GDT is being used after load_direct_gdt() */
> +		if (within_area(addr, end, (unsigned long)get_cpu_gdt_rw(cpu),
> +				GDT_SIZE))

... why the O(n) loop over the system?

It is only GDTs which might ever be active on this local CPU(/thread)
which are a problem, because the breakpoint registers are similarly local.

Nothing is going to go wrong If I put a breakpoint on someone else's
live GDT, because they wont interact in the "fun" ways we're trying to
avoid.

~Andrew

  reply index

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-29 21:27 [PATCH 00/14] x86/entry: disallow #DB more and x86/entry lockdep/nmi Peter Zijlstra
2020-05-29 21:27 ` [PATCH 01/14] x86/hw_breakpoint: Add within_area() to check data breakpoints Peter Zijlstra
2020-05-29 21:27 ` [PATCH 02/14] x86/hw_breakpoint: Prevent data breakpoints on direct GDT Peter Zijlstra
2020-05-30 12:45   ` Andrew Cooper [this message]
2020-05-30 15:15     ` Lai Jiangshan
2020-05-29 21:27 ` [PATCH 03/14] x86/hw_breakpoint: Prevent data breakpoints on per_cpu cpu_tss_rw Peter Zijlstra
2020-05-29 21:27 ` [PATCH 04/14] x86/hw_breakpoint: Prevent data breakpoints on user_pcid_flush_mask Peter Zijlstra
2020-05-29 21:27 ` [PATCH 05/14] x86/entry: Introduce local_db_{save,restore}() Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 06/14] x86/entry, nmi: Disable #DB Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 07/14] x86/entry, mce: Disallow #DB during #MC Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 08/14] x86/entry: Optimize local_db_save() for virt Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-06-03  1:17   ` [PATCH 08/14] " Sean Christopherson
2020-05-29 21:27 ` [PATCH 09/14] x86/entry: Remove debug IDT frobbing Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 10/14] x86/entry: Remove DBn stacks Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 11/14] x86/entry: Clarify irq_{enter,exit}_rcu() Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-06-02 14:42   ` [PATCH 11/14] " Qian Cai
2020-06-02 15:05     ` Peter Zijlstra
2020-06-02 18:47       ` Qian Cai
2020-06-03 17:50       ` [tip: x86/entry] x86/entry: Use __irq_exit_rcu() in irq_exit() tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 12/14] x86/entry: Rename trace_hardirqs_off_prepare() Peter Zijlstra
2020-05-30  9:57   ` [tip: x86/entry] " tip-bot2 for Peter Zijlstra
2020-05-29 21:27 ` [PATCH 13/14] lockdep: Prepare for NMI IRQ state tracking Peter Zijlstra
2020-05-29 22:14   ` Steven Rostedt
2020-05-29 22:25     ` Peter Zijlstra
2020-05-29 22:28       ` Steven Rostedt
2020-05-29 22:33       ` Peter Zijlstra
2020-06-02 20:00       ` Peter Zijlstra
2020-05-29 21:27 ` [PATCH 14/14] x86/entry: Fix NMI vs " Peter Zijlstra

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