Re: [PATCH v3 1/8] powerpc/uaccess: Add unsafe_copy_from_user

["Christopher M. Riedl" <cmr@codefail.de>]

On Mon Jan 11, 2021 at 7:22 AM CST, Christophe Leroy wrote:
>
>
> Le 09/01/2021 à 04:25, Christopher M. Riedl a écrit :
> > Implement raw_copy_from_user_allowed() which assumes that userspace read
> > access is open. Use this new function to implement raw_copy_from_user().
> > Finally, wrap the new function to follow the usual "unsafe_" convention
> > of taking a label argument.
>
> I think there is no point implementing raw_copy_from_user_allowed(), see
> https://github.com/linuxppc/linux/commit/4b842e4e25b1 and
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/8c74fc9ce8131cabb10b3e95dc0e430f396ee83e.1610369143.git.christophe.leroy@csgroup.eu/
>
> You should simply do:
>
> #define unsafe_copy_from_user(d, s, l, e) \
> unsafe_op_wrap(__copy_tofrom_user((__force void __user *)d, s, l), e)
>

I gave this a try and the signal ops decreased by ~8K. Now, to be
honest, I am not sure what an "acceptable" benchmark number here
actually is - so maybe this is ok? Same loss with both radix and hash:

	|                                      | hash   | radix  |
	| ------------------------------------ | ------ | ------ |
	| linuxppc/next                        | 118693 | 133296 |
	| linuxppc/next w/o KUAP+KUEP          | 228911 | 228654 |
	| unsafe-signal64                      | 200480 | 234067 |
	| unsafe-signal64 (__copy_tofrom_user) | 192467 | 225119 |

To put this into perspective, prior to KUAP and uaccess flush, signal
performance in this benchmark was ~290K on hash.

>
> Christophe
>
> > 
> > The new raw_copy_from_user_allowed() calls non-inline __copy_tofrom_user()
> > internally. This is still safe to call inside user access blocks formed
> > with user_*_access_begin()/user_*_access_end() since asm functions are not
> > instrumented for tracing.
> > 
> > Signed-off-by: Christopher M. Riedl <cmr@codefail.de>
> > ---
> >   arch/powerpc/include/asm/uaccess.h | 28 +++++++++++++++++++---------
> >   1 file changed, 19 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> > index 501c9a79038c..698f3a6d6ae5 100644
> > --- a/arch/powerpc/include/asm/uaccess.h
> > +++ b/arch/powerpc/include/asm/uaccess.h
> > @@ -403,38 +403,45 @@ raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
> >   }
> >   #endif /* __powerpc64__ */
> >   
> > -static inline unsigned long raw_copy_from_user(void *to,
> > -		const void __user *from, unsigned long n)
> > +static inline unsigned long
> > +raw_copy_from_user_allowed(void *to, const void __user *from, unsigned long n)
> >   {
> > -	unsigned long ret;
> >   	if (__builtin_constant_p(n) && (n <= 8)) {
> > -		ret = 1;
> > +		unsigned long ret = 1;
> >   
> >   		switch (n) {
> >   		case 1:
> >   			barrier_nospec();
> > -			__get_user_size(*(u8 *)to, from, 1, ret);
> > +			__get_user_size_allowed(*(u8 *)to, from, 1, ret);
> >   			break;
> >   		case 2:
> >   			barrier_nospec();
> > -			__get_user_size(*(u16 *)to, from, 2, ret);
> > +			__get_user_size_allowed(*(u16 *)to, from, 2, ret);
> >   			break;
> >   		case 4:
> >   			barrier_nospec();
> > -			__get_user_size(*(u32 *)to, from, 4, ret);
> > +			__get_user_size_allowed(*(u32 *)to, from, 4, ret);
> >   			break;
> >   		case 8:
> >   			barrier_nospec();
> > -			__get_user_size(*(u64 *)to, from, 8, ret);
> > +			__get_user_size_allowed(*(u64 *)to, from, 8, ret);
> >   			break;
> >   		}
> >   		if (ret == 0)
> >   			return 0;
> >   	}
> >   
> > +	return __copy_tofrom_user((__force void __user *)to, from, n);
> > +}
> > +
> > +static inline unsigned long
> > +raw_copy_from_user(void *to, const void __user *from, unsigned long n)
> > +{
> > +	unsigned long ret;
> > +
> >   	barrier_nospec();
> >   	allow_read_from_user(from, n);
> > -	ret = __copy_tofrom_user((__force void __user *)to, from, n);
> > +	ret = raw_copy_from_user_allowed(to, from, n);
> >   	prevent_read_from_user(from, n);
> >   	return ret;
> >   }
> > @@ -542,6 +549,9 @@ user_write_access_begin(const void __user *ptr, size_t len)
> >   #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
> >   #define unsafe_put_user(x, p, e) __put_user_goto(x, p, e)
> >   
> > +#define unsafe_copy_from_user(d, s, l, e) \
> > +	unsafe_op_wrap(raw_copy_from_user_allowed(d, s, l), e)
> > +
> >   #define unsafe_copy_to_user(d, s, l, e) \
> >   do {									\
> >   	u8 __user *_dst = (u8 __user *)(d);				\
> >