linux-kernel - Re: [PATCH v2 06/15] powerpc/align: Don't use __get_user

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Message-ID: <877dlukhmi.fsf@dja-thinkpad.axtens.net>
Date:   Fri, 26 Mar 2021 09:12:21 +1100
From:   Daniel Axtens <dja@...ens.net>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 06/15] powerpc/align: Don't use __get_user_instr() on kernel addresses

Christophe Leroy <christophe.leroy@...roup.eu> writes:

> In the old days, when we didn't have kernel userspace access
> protection and had set_fs(), it was wise to use __get_user()
> and friends to read kernel memory.
>
> Nowadays, get_user() is granting userspace access and is exclusively
> for userspace access.
>
> In alignment exception handler, use probe_kernel_read_inst()
> instead of __get_user_instr() for reading instructions in kernel.
>
> This will allow to remove the is_kernel_addr() check in
> __get/put_user() in a following patch.
>

Looks good to me!

Reviewed-by: Daniel Axtens <dja@...ens.net>

Kind regards,
Daniel

> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/powerpc/kernel/align.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/align.c b/arch/powerpc/kernel/align.c
> index c4d7b445b459..8d4c7af262e2 100644
> --- a/arch/powerpc/kernel/align.c
> +++ b/arch/powerpc/kernel/align.c
> @@ -310,7 +310,11 @@ int fix_alignment(struct pt_regs *regs)
>  	 */
>  	CHECK_FULL_REGS(regs);
>  
> -	if (unlikely(__get_user_instr(instr, (void __user *)regs->nip)))
> +	if (is_kernel_addr(regs->nip))
> +		r = probe_kernel_read_inst(&instr, (void *)regs->nip);
> +	else
> +		r = __get_user_instr(instr, (void __user *)regs->nip);
> +	if (unlikely(r))
>  		return -EFAULT;
>  	if ((regs->msr & MSR_LE) != (MSR_KERNEL & MSR_LE)) {
>  		/* We don't handle PPC little-endian any more... */
> -- 
> 2.25.0