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

[Mark Rutland]

On Sat, Jun 18, 2022 at 05:27:45PM +0800, Tong Tiangen wrote:
> 
> 
> 在 2022/6/17 17:06, Mark Rutland 写道:
> > On Sat, May 28, 2022 at 06:50:55AM +, 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 
> > 
> > > ---
> > >   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.

Cool, thanks. If this patch just has the extable change, that's fine by me.

> > 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().

I don't think that's true.

IIUC per the last patch, in the exception handler we'll apply the fixup then
force a signal. That doesn't kill the task immediately, and we'll return from
the exception handler back into the original context (with the fixup applied).

The structure of copy_from_user() is 

copy_from_user(to, from, n) {
_copy_from_user(to, from, n) {
res = n;
res = raw_copy_from_user(to, from, n);
if (res) 
memset(to + (n - res), 0, res);
}
}

So when the fixup is applied and res indicates that the copy terminated early,
there is an unconditinal memset() before the fatal signal is handled in the
return to userspace path.

> > 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.

[...]

> > > 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?

As above, if you just retain the extable.c changes, that's fine by me.

Thanks,
Mark.

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

[Mark Rutland]

On Sat, May 28, 2022 at 06:50:55AM +, 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 

> ---
>  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.

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 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  .reqx5
> 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?

Thanks,
Mark.