linux-kernel - Re: [RFC PATCH -next V3 4/6] arm64: add copy_{to, from}

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Message-ID: <306b1b09-487a-9ccd-4a63-8c78889492c6@arm.com>
Date:   Tue, 12 Apr 2022 18:17:58 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Tong Tiangen <tongtiangen@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Kefeng Wang <wangkefeng.wang@...wei.com>,
        Xie XiuQi <xiexiuqi@...wei.com>
Subject: Re: [RFC PATCH -next V3 4/6] arm64: add copy_{to, from}_user to
 machine check safe

On 12/04/2022 6:08 pm, Robin Murphy wrote:
[...]
>> @@ -62,7 +63,11 @@ SYM_FUNC_START(__arch_copy_from_user)
>>       ret
>>       // Exception fixups
>> -9997:    cmp    dst, dstin
>> +9997:    mrs esr, esr_el1            // Check exception first
>> +    and esr, esr, #ESR_ELx_FSC
>> +    cmp esr, #ESR_ELx_FSC_EXTABT
> 
> Should we be checking EC to make sure it's a data abort - and thus FSC 
> is valid - in the first place? I'm a little fuzzy on all the possible 
> paths into fixup_exception(), and it's not entirely obvious whether this 
> is actually safe or not.

In fact, thinking some more about that, I don't think there should be 
any need for this sort of logic in these handlers at all. The 
fixup_exception() machinery should already know enough about the 
exception that's happened and the extable entry to figure this out and 
not bother calling the handler at all.

Thanks,
Robin.