Re: [PATCH -next v5 7/8] arm64: add uaccess to machine check safe

[Tong Tiangen <tongtiangen@xxxxxxxxxx>]

在 2022/6/17 17:06, Mark Rutland 写道:
> On Sat, May 28, 2022 at 06:50:55AM +0000, Tong Tiangen wrote:
> > If user access fail due to hardware memory error, only the relevant
> > processes are affected, so killing the user process and isolate the
> > error page with hardware memory errors is a more reasonable choice
> > than kernel panic.
> >
> > Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>
>
> > ---
> >   arch/arm64/lib/copy_from_user.S | 8 ++++----
> >   arch/arm64/lib/copy_to_user.S   | 8 ++++----
>
> All of these changes are to the *kernel* accesses performed as part of copy
> to/from user, and have nothing to do with userspace, so it does not make sense
> to mark these as UACCESS.

You have a point. so there is no need to modify copy_from/to_user.S in 
this patch set.

> Do we *actually* need to recover from failues on these accesses? Looking at
> _copy_from_user(), the kernel will immediately follow this up with a memset()
> to the same address which will be fatal anyway, so this is only punting the
> failure for a few instructions.

If recovery success, The task will be killed and there will be no 
subsequent memset().

> If we really need to recover from certain accesses to kernel memory we should
> add a new EX_TYPE_KACCESS_ERR_ZERO_MC or similar, but we need a strong
> rationale as to why that's useful. As things stand I do not beleive it makes
> sense for copy to/from user specifically.
>
> >   arch/arm64/mm/extable.c         | 8 ++++----
> >   3 files changed, 12 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
> > index 34e317907524..402dd48a4f93 100644
> > --- a/arch/arm64/lib/copy_from_user.S
> > +++ b/arch/arm64/lib/copy_from_user.S
> > @@ -25,7 +25,7 @@
> >   	.endm
> >   
> >   	.macro strb1 reg, ptr, val
> > -	strb \reg, [\ptr], \val
> > +	USER(9998f, strb \reg, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro ldrh1 reg, ptr, val
> > @@ -33,7 +33,7 @@
> >   	.endm
> >   
> >   	.macro strh1 reg, ptr, val
> > -	strh \reg, [\ptr], \val
> > +	USER(9998f, strh \reg, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro ldr1 reg, ptr, val
> > @@ -41,7 +41,7 @@
> >   	.endm
> >   
> >   	.macro str1 reg, ptr, val
> > -	str \reg, [\ptr], \val
> > +	USER(9998f, str \reg, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro ldp1 reg1, reg2, ptr, val
> > @@ -49,7 +49,7 @@
> >   	.endm
> >   
> >   	.macro stp1 reg1, reg2, ptr, val
> > -	stp \reg1, \reg2, [\ptr], \val
> > +	USER(9998f, stp \reg1, \reg2, [\ptr], \val)
> >   	.endm
> >   
> >   end	.req	x5
> > diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
> > index 802231772608..4134bdb3a8b0 100644
> > --- a/arch/arm64/lib/copy_to_user.S
> > +++ b/arch/arm64/lib/copy_to_user.S
> > @@ -20,7 +20,7 @@
> >    *	x0 - bytes not copied
> >    */
> >   	.macro ldrb1 reg, ptr, val
> > -	ldrb  \reg, [\ptr], \val
> > +	USER(9998f, ldrb  \reg, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro strb1 reg, ptr, val
> > @@ -28,7 +28,7 @@
> >   	.endm
> >   
> >   	.macro ldrh1 reg, ptr, val
> > -	ldrh  \reg, [\ptr], \val
> > +	USER(9998f, ldrh  \reg, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro strh1 reg, ptr, val
> > @@ -36,7 +36,7 @@
> >   	.endm
> >   
> >   	.macro ldr1 reg, ptr, val
> > -	ldr \reg, [\ptr], \val
> > +	USER(9998f, ldr \reg, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro str1 reg, ptr, val
> > @@ -44,7 +44,7 @@
> >   	.endm
> >   
> >   	.macro ldp1 reg1, reg2, ptr, val
> > -	ldp \reg1, \reg2, [\ptr], \val
> > +	USER(9998f, ldp \reg1, \reg2, [\ptr], \val)
> >   	.endm
> >   
> >   	.macro stp1 reg1, reg2, ptr, val
> > diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> > index c301dcf6335f..8ca8d9639f9f 100644
> > --- a/arch/arm64/mm/extable.c
> > +++ b/arch/arm64/mm/extable.c
> > @@ -86,10 +86,10 @@ bool fixup_exception_mc(struct pt_regs *regs)
> >   	if (!ex)
> >   		return false;
> >   
> > -	/*
> > -	 * This is not complete, More Machine check safe extable type can
> > -	 * be processed here.
> > -	 */
> > +	switch (ex->type) {
> > +	case EX_TYPE_UACCESS_ERR_ZERO:
> > +		return ex_handler_uaccess_err_zero(ex, regs);
> > +	}
>
> This addition specifically makes sense to me, so can you split this into a separate patch?

According to my understanding of the above, only the modification of 
extable.c is retained.

So what do you mean which part is made into a separate patch?

Thanks,
Tong.
> Thanks,
> Mark.
>
> .