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Subject: Re: [PATCH] [V2] x86/asm: add __user on copy_user_handle_tail()
 pointers
To:     Ben Dooks <ben.dooks@codethink.co.uk>, linux-kernel@vger.kernel.org
Cc:     x86@kernel.org, mingo@redhat.co, bp@alien8.de, hpa@zytor.com,
        torvalds@linux-foundation.org, tglx@linutronix.de
References: <20190330115624.4000-1-ben.dooks@codethink.co.uk>
From:   Mukesh Ojha <mojha@codeaurora.org>
Message-ID: <65f52f59-3513-bf53-cb73-36bad40fad2d@codeaurora.org>
Date:   Sun, 31 Mar 2019 01:05:28 +0530
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On 3/30/2019 5:26 PM, Ben Dooks wrote:
> The copy_user_handle_tail() clearly uses both from and to as pointers
> to user-space memory. This triggers sparse warning on using the calls
> to get and put to user-space. This can be fixed easily by changing the
> call to take __user annotated pointer.s
>
> arch/x86/lib/usercopy_64.c:68:21: warning: incorrect type in argument 1 (different address spaces)
> arch/x86/lib/usercopy_64.c:68:21:    expected void const volatile [noderef] <asn:1>*<noident>
> arch/x86/lib/usercopy_64.c:68:21:    got char *
> arch/x86/lib/usercopy_64.c:70:21: warning: incorrect type in argument 1 (different address spaces)
> arch/x86/lib/usercopy_64.c:70:21:    expected void const volatile [noderef] <asn:1>*<noident>
> arch/x86/lib/usercopy_64.c:70:21:    got char *to
>
>  From Linus Torvalds:
>
> On Thu, Mar 28, 2019 at 12:24 AM Borislav Petkov <bp@alien8.de> wrote:
>
> Well, but copy_user_generic() (which ends up calling the
> copy_user_handle_tail() eventually) casts those __user pointers to
> (__force void *). Converting them back to __user looks strange to me.
>
> Linus?
>
> Well, it does that because the x86 version of copy_user_generic() can
> work in either direction, so it works when either the source or
> destination (or both) are user pointers, but they don't _have_ to be.
>
> So the "userness" of a pointer in that context is a bit ambiguous, and
> so we've picked the pointers to be just plain "void *".
>
> That said, arguably we should have gone the other way and just made
> them both "__user" pointers, and do the cast the other way around.
>
> But there's no absolutely right answer here, and nobody should ever
> use copy_user_generic() directly (ie it is very much meant to be only
> used as a internal helper for the cases that get the pointer
> annotations right).
>
> Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>

Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>

Cheers,
-Mukesh


> ---
>   arch/x86/include/asm/uaccess_64.h | 2 +-
>   arch/x86/lib/usercopy_64.c        | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
> index a9d637bc301d..cbca2cb28939 100644
> --- a/arch/x86/include/asm/uaccess_64.h
> +++ b/arch/x86/include/asm/uaccess_64.h
> @@ -208,7 +208,7 @@ __copy_from_user_flushcache(void *dst, const void __user *src, unsigned size)
>   }
>   
>   unsigned long
> -copy_user_handle_tail(char *to, char *from, unsigned len);
> +copy_user_handle_tail(char __user *to, char __user *from, unsigned len);
>   
>   unsigned long
>   mcsafe_handle_tail(char *to, char *from, unsigned len);
> diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
> index ee42bb0cbeb3..aa180424e77a 100644
> --- a/arch/x86/lib/usercopy_64.c
> +++ b/arch/x86/lib/usercopy_64.c
> @@ -60,7 +60,7 @@ EXPORT_SYMBOL(clear_user);
>    * it is not necessary to optimize tail handling.
>    */
>   __visible unsigned long
> -copy_user_handle_tail(char *to, char *from, unsigned len)
> +copy_user_handle_tail(char __user *to, char __user *from, unsigned len)
>   {
>   	for (; len; --len, to++) {
>   		char c;