linux-kernel - Re: [PATCH v11 4/5] arm64: mte: Enable async tag check fault

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Date:   Fri, 5 Feb 2021 15:45:38 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH v11 4/5] arm64: mte: Enable async tag check fault



On 2/5/21 3:39 PM, Catalin Marinas wrote:
> On Sat, Jan 30, 2021 at 04:52:24PM +0000, Vincenzo Frascino wrote:
>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index 92078e1eb627..7763ac1f2917 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -182,6 +182,37 @@ bool mte_report_once(void)
>>  	return READ_ONCE(report_fault_once);
>>  }
>>  
>> +#ifdef CONFIG_KASAN_HW_TAGS
>> +void mte_check_tfsr_el1(void)
>> +{
>> +	u64 tfsr_el1;
>> +
>> +	if (!system_supports_mte())
>> +		return;
>> +
>> +	tfsr_el1 = read_sysreg_s(SYS_TFSR_EL1);
>> +
>> +	/*
>> +	 * The kernel should never trigger an asynchronous fault on a
>> +	 * TTBR0 address, so we should never see TF0 set.
>> +	 * For futexes we disable checks via PSTATE.TCO.
>> +	 */
>> +	WARN_ONCE(tfsr_el1 & SYS_TFSR_EL1_TF0,
>> +		  "Kernel async tag fault on TTBR0 address");
> 
> Sorry, I got confused when I suggested this warning. If the user is
> running in async mode, the TFSR_EL1.TF0 bit may be set by
> copy_mount_options(), strncpy_from_user() which rely on an actual fault
> happening (not the case with asynchronous where only a bit is set). With
> the user MTE support, we never report asynchronous faults caused by the
> kernel on user addresses as we can't easily track them. So this warning
> may be triggered on correctly functioning kernel/user.
> 

No issue, I will re-post removing the WARN_ONCE().

>> +
>> +	if (unlikely(tfsr_el1 & SYS_TFSR_EL1_TF1)) {
>> +		/*
>> +		 * Note: isb() is not required after this direct write
>> +		 * because there is no indirect read subsequent to it
>> +		 * (per ARM DDI 0487F.c table D13-1).
>> +		 */
>> +		write_sysreg_s(0, SYS_TFSR_EL1);
> 
> Zeroing the whole register is still fine, we don't care about the TF0
> bit anyway.
> 
>> +
>> +		kasan_report_async();
>> +	}
>> +}
>> +#endif
> 

-- 
Regards,
Vincenzo