linux-kernel - Re: [PATCH v3 5/5] x86/sgx: Remove redundant if conditions in sgx_encl

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Message-ID: <YA8SAf3RFTGU/mhL@google.com>
Date:   Mon, 25 Jan 2021 10:46:25 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jia Zhang <zhang.jia@...ux.alibaba.com>
Subject: Re: [PATCH v3 5/5] x86/sgx: Remove redundant if conditions in
 sgx_encl_create

On Sun, Jan 24, 2021, Greg KH wrote:
> On Sun, Jan 24, 2021 at 02:29:07PM +0800, Tianjia Zhang wrote:
> > In this scenario, there is no case where va_page is NULL, and
> > the error has been checked. The if condition statement here is
> > redundant, so remove the condition detection.
> > 
> > Signed-off-by: Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
> > ---
> >  arch/x86/kernel/cpu/sgx/ioctl.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
> > index 1c6ecf9fbeff..b0b829f1b761 100644
> > --- a/arch/x86/kernel/cpu/sgx/ioctl.c
> > +++ b/arch/x86/kernel/cpu/sgx/ioctl.c
> > @@ -66,9 +66,11 @@ static int sgx_encl_create(struct sgx_encl *encl, struct sgx_secs *secs)
> >  	va_page = sgx_encl_grow(encl);
> >  	if (IS_ERR(va_page))
> >  		return PTR_ERR(va_page);
> > -	else if (va_page)
> > -		list_add(&va_page->list, &encl->va_pages);
> > -	/* else the tail page of the VA page list had free slots. */
> > +
> > +	if (WARN_ONCE(!va_page, "non-empty VA page list before ECREATE"))
> > +		return -EIO;
> 
> So you just crashed machines that have panic-on-warn enabled.  Don't do
> that for no reason, just handle the error and move on.

The WARN will only fire if someone introduces a kernel bug.  It's one part
documentation, two parts helping detect future breakage.  Even if the VA page
management is significantly reworked, I'm having a hard time envisioning a
scenario where adding VA pages before ECREATE would be anything but a kernel bug.