Re: [PATCH v3 5/8] arm64: Fix incorrect irqflag restore for priority masking

[Marc Zyngier]

On 10/06/2019 12:36, Julien Thierry wrote:
> 
> 
> On 10/06/2019 08:49, Julien Thierry wrote:
>>
>>
>> On 07/06/2019 17:29, Marc Zyngier wrote:
>>> On 06/06/2019 10:31, Julien Thierry wrote:
>>>> When using IRQ priority masking to disable interrupts, in order to deal
>>>> with the PSR.I state, local_irq_save() would convert the I bit into a
>>>> PMR value (GIC_PRIO_IRQOFF). This resulted in local_irq_restore()
>>>> potentially modifying the value of PMR in undesired location due to the
>>>> state of PSR.I upon flag saving [1].
>>>>
>>>> In an attempt to solve this issue in a less hackish manner, introduce
>>>> a bit (GIC_PRIO_IGNORE_PMR) for the PMR values that can represent
>>>> whether PSR.I is being used to disable interrupts, in which case it
>>>> takes precedence of the status of interrupt masking via PMR.
>>>>
>>>> GIC_PRIO_IGNORE_PMR is chosen such that (<pmr_value> |
>>>> GIC_PRIO_IGNORE_PMR) does not mask more interrupts than <pmr_value> as
>>>> some sections (e.g. arch_cpu_idle(), interrupt acknowledge path)
>>>> requires PMR not to mask interrupts that could be signaled to the
>>>> CPU when using only PSR.I.
>>>>
>>>
>>> s/GIC_PRIO_IGNORE_PMR/GIC_PRIO_PSR_I_SET/
>>>
>>>> [1] https://www.spinics.net/lists/arm-kernel/msg716956.html
>>>>
>>>> Fixes: commit 4a503217ce37 ("arm64: irqflags: Use ICC_PMR_EL1 for interrupt masking")
>>>> Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
>>>> Reported-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
>>>> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
>>>> Cc: Wei Li <liwei391@xxxxxxxxxx>
>>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>>> Cc: Will Deacon <will.deacon@xxxxxxx>
>>>> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
>>>> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>>>> Cc: James Morse <james.morse@xxxxxxx>
>>>> Cc: Suzuki K Pouloze <suzuki.poulose@xxxxxxx>
>>>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>>>> ---
>>>>  arch/arm64/include/asm/arch_gicv3.h |  4 ++-
>>>>  arch/arm64/include/asm/daifflags.h  | 68 ++++++++++++++++++++++---------------
>>>>  arch/arm64/include/asm/irqflags.h   | 67 +++++++++++++++---------------------
>>>>  arch/arm64/include/asm/kvm_host.h   |  7 ++--
>>>>  arch/arm64/include/asm/ptrace.h     | 10 ++++--
>>>>  arch/arm64/kernel/entry.S           | 38 ++++++++++++++++++---
>>>>  arch/arm64/kernel/process.c         |  2 +-
>>>>  arch/arm64/kernel/smp.c             |  8 +++--
>>>>  arch/arm64/kvm/hyp/switch.c         |  2 +-
>>>>  9 files changed, 123 insertions(+), 83 deletions(-)
>>>>
> 
> [...]
> 
>>>> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
>>>> index fbe1aba..b6f757f 100644
>>>> --- a/arch/arm64/include/asm/irqflags.h
>>>> +++ b/arch/arm64/include/asm/irqflags.h
>>>> @@ -67,43 +67,46 @@ static inline void arch_local_irq_disable(void)
>>>>   */
>>>>  static inline unsigned long arch_local_save_flags(void)
>>>>  {
>>>> -	unsigned long daif_bits;
>>>>  	unsigned long flags;
>>>>
>>>> -	daif_bits = read_sysreg(daif);
>>>> -
>>>> -	/*
>>>> -	 * The asm is logically equivalent to:
>>>> -	 *
>>>> -	 * if (system_uses_irq_prio_masking())
>>>> -	 *	flags = (daif_bits & PSR_I_BIT) ?
>>>> -	 *			GIC_PRIO_IRQOFF :
>>>> -	 *			read_sysreg_s(SYS_ICC_PMR_EL1);
>>>> -	 * else
>>>> -	 *	flags = daif_bits;
>>>> -	 */
>>>>  	asm volatile(ALTERNATIVE(
>>>> -			"mov	%0, %1\n"
>>>> -			"nop\n"
>>>> -			"nop",
>>>> -			__mrs_s("%0", SYS_ICC_PMR_EL1)
>>>> -			"ands	%1, %1, " __stringify(PSR_I_BIT) "\n"
>>>> -			"csel	%0, %0, %2, eq",
>>>> -			ARM64_HAS_IRQ_PRIO_MASKING)
>>>> -		: "=&r" (flags), "+r" (daif_bits)
>>>> -		: "r" ((unsigned long) GIC_PRIO_IRQOFF)
>>>> -		: "cc", "memory");
>>>> +		"mrs	%0, daif",
>>>> +		__mrs_s("%0", SYS_ICC_PMR_EL1),
>>>> +		ARM64_HAS_IRQ_PRIO_MASKING)
>>>> +		: "=&r" (flags)
>>>> +		:
>>>> +		: "memory");
>>>
>>> I think this is worth a comment here, as you're changing the semantics
>>> of arch_local_save_flags(). Maybe just indicating that the only thing
>>> this should be used for is to carry the interrupt state, and nothing else.
>>>
>>
>> Arguably, this is what gets called by local_save_flags() which is arch
>> independent and, as far as I understand, is only aware of the interrupt
>> state being contained in the flags (arch might wish to store more stuff
>> in it, but overall, generic code cannot rely on it).
>>
>> I'll still add a comment so that code directly calling arch_save_flags()
>> doesn't try to play with PSR.DA_F. (In such a cases it would be probably
>> clearer for them to do direct DAIF reads/writes IMO).
>>
> 
> After checking, arch_local_save_flags() already has the following
> comment above it:
> 
> /*
> 
> 
> 
>  * Save the current interrupt enable state.
> 
> 
> 
>  */
> 
> 
> Which suggests you shouldn't rely on having the value of debug state and
> other (it just happens to be there, maybe wrongfully...).
> 
> And user checking the flags should use arch_irqs_disabled_flags() rather
> than "flags & PSR_I_BIT != 0".
> 
> Also, those semantics where already changed when we introduced priority
> masking and included the PMR value in the irqflags.
> 
> I'm not sure there is a lot more explanation to do in this patch in
> particular.

Fair enough. I guess that if someone is fiddling with the flags in
ungodly ways, they deserve to be bitten...

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...