Re: [PATCH v2 7/8] x86/insn: Extend error reporting from insn_fetch_from_user[_inatomic]()

From: Borislav Petkov <bp@alien8.de>
To: Joerg Roedel <joro@8bytes.org>
Cc: x86@kernel.org, Hyunwook Baek <baekhw@google.com>,
	Joerg Roedel <jroedel@suse.de>,
	hpa@zytor.com, Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Juergen Gross <jgross@suse.com>,
	Kees Cook <keescook@chromium.org>,
	David Rientjes <rientjes@google.com>,
	Cfir Cohen <cfir@google.com>, Erdem Aktas <erdemaktas@google.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v2 7/8] x86/insn: Extend error reporting from insn_fetch_from_user[_inatomic]()
Date: Fri, 21 May 2021 16:34:34 +0200	[thread overview]
Message-ID: <YKfE+gfK5AdE9ckm@zn.tnic> (raw)
In-Reply-To: <20210519135251.30093-8-joro@8bytes.org>

On Wed, May 19, 2021 at 03:52:50PM +0200, Joerg Roedel wrote:
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 4eecb9c7c6a0..d8a057ba0895 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -1442,27 +1442,36 @@ static int insn_get_effective_ip(struct pt_regs *regs, unsigned long *ip)
>   * insn_fetch_from_user() - Copy instruction bytes from user-space memory
>   * @regs:	Structure with register values as seen when entering kernel mode
>   * @buf:	Array to store the fetched instruction
> + * @copied:	Pointer to an int where the number of copied instruction bytes
> + *		is stored. Can be NULL.
>   *
>   * Gets the linear address of the instruction and copies the instruction bytes
>   * to the buf.
>   *
>   * Returns:
>   *
> - * Number of instruction bytes copied.
> + * -EINVAL if the linear address of the instruction could not be calculated
> + * -EFAULT if nothing was copied
> + *       0 on success
>   *
> - * 0 if nothing was copied.
>   */
> -int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
> +int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE],
> +			 int *copied)
>  {
>  	unsigned long ip;
>  	int not_copied;
> +	int bytes;
>  
>  	if (insn_get_effective_ip(regs, &ip))
> -		return 0;
> +		return -EINVAL;
>  
>  	not_copied = copy_from_user(buf, (void __user *)ip, MAX_INSN_SIZE);
>  
> -	return MAX_INSN_SIZE - not_copied;
> +	bytes = MAX_INSN_SIZE - not_copied;
> +	if (copied)
> +		*copied = bytes;
> +
> +	return bytes ? 0 : -EFAULT;

Why not simpler?

return value >= 0 says how many bytes were copied
return value < 0 means some kind of error

And then you don't need @copied...

Ditto for the other one.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette