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

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Date:   Sat, 16 Apr 2022 15:41:09 +0800
From:   Tong Tiangen <tongtiangen@...wei.com>
To:     Robin Murphy <robin.murphy@....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



在 2022/4/13 1:17, Robin Murphy 写道:
> 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.
> .

Hi Robin:
As you said, it seems that it's not good to judge esr here, how about 
using the following method, i need your suggestion :)

+#define FIXUP_TYPE_NORMAL	0
+#define FIXUP_TYPE_MC		1

arch/arm64/mm/extable.c
static bool ex_handler_fixup(const struct exception_table_entry *ex,
-	struct pt_regs *regs)
+	struct pt_regs *regs, int fixuptype)
{
+	regs->regs[16] = fixuptype;
	[...]
}

bool fixup_exception(struct pt_regs *regs)
{
	[...]
	switch(ex->type) {
	case EX_TYPE_UACCESS_MC:
-		return ex_handler_fixup(ex, regs)
+		return ex_handler_fixup(ex, regs, FIXUP_TYPE_NORMAL)
	break;
	}
	[...]
}

bool fixup_exception_mc(struct pt_regs *regs)
{
	[...]
	switch(ex->type) {
	case EX_TYPE_UACCESS_MC:
-		return ex_handler_fixup(ex, regs)
+		return ex_handler_fixup(ex, regs, FIXUP_TYPE_MC)
	break;
	}
	[...]
}

arch/arm64/lib/copy_from_user.S
arch/arm64/lib/copy_to_user.S

+fixup_type      .req    x16

// Exception fixups
//x16: fixup type written by ex_handler_fixup
-9997:  cmp     dst, dstin
+9997:	cmp fixup_type, #FIXUP_TYPE_MC
+	b.eq 9998f
+ 	cmp     dst, dstin
  	b.ne    9998f

Thanks,
Tong.