linux-kernel - Re: [PATCH v2 3/4] x86/fpu: Fix _

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Date:   Fri, 29 Mar 2019 19:03:31 +0100
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Qiaowei Ren <qiaowei.ren@...el.com>
Subject: Re: [PATCH v2 3/4] x86/fpu: Fix __user annotations

On Fri, Mar 29, 2019 at 05:30:46PM +0100, Jann Horn wrote:
> In save_xstate_epilog(), use __user when type-casting userspace pointers.
> 
> In setup_sigcontext() and x32_setup_rt_frame(), perform explicit __force
> casts for converting userspace pointers to unsigned long; put_user_ex()
> already performs a cast, but without __force, which is required by sparse
> for conversions from userspace pointers to numbers.

...
 
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 08dfd4c1a4f9..e13cd972f9af 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -206,7 +206,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate,
>  		put_user_ex(regs->ss, &sc->ss);
>  #endif /* CONFIG_X86_32 */
>  
> -		put_user_ex(fpstate, &sc->fpstate);
> +		put_user_ex((unsigned long __force)fpstate, &sc->fpstate);

The __force here is not needed and in fact meaningless as the address
space annotations and checks only concern pointers. By casting a
pointer to an unsigned long, all type info is lost anyway and thus
no address-space checks are performed. It's a bit like such casts
always have an implicit __force already included.

> @@ -569,7 +569,7 @@ static int x32_setup_rt_frame(struct ksignal *ksig,
>  			restorer = NULL;
>  			err |= -EFAULT;
>  		}
> -		put_user_ex(restorer, &frame->pretcode);
> +		put_user_ex((unsigned long __force)restorer, &frame->pretcode);

Same here.

Best regards,
-- Luc Van Oostenryck