Re: [RFC PATCH -next V3 4/6] arm64: add copy_{to, from}_user to machine check safe

From: Robin Murphy <robin.murphy@arm.com>
To: Tong Tiangen <tongtiangen@huawei.com>,
	Mark Rutland <mark.rutland@arm.com>,
	James Morse <james.morse@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>
Cc: linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Kefeng Wang <wangkefeng.wang@huawei.com>,
	Xie XiuQi <xiexiuqi@huawei.com>
Subject: Re: [RFC PATCH -next V3 4/6] arm64: add copy_{to, from}_user to machine check safe
Date: Tue, 12 Apr 2022 18:17:58 +0100	[thread overview]
Message-ID: <306b1b09-487a-9ccd-4a63-8c78889492c6@arm.com> (raw)
In-Reply-To: <38c6d4b5-a3db-5c3e-02e7-39875edb3476@arm.com>

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.