Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 18 Aug 2022 13:06:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Pengfei Xu <pengfei.xu@intel.com>
Cc:     Josh Poimboeuf <jpoimboe@kernel.org>,
        "Yang, Weijiang" <weijiang.yang@intel.com>,
        "Su, Heng" <heng.su@intel.com>, linux-kernel@vger.kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>, pbonzini@redhat.com,
        x86@kernel.org
Subject: Re: [PATCH] x86/kvm, objtool: Avoid fastop ENDBR from being sealed
Message-ID: <Yv4dKABja9CxqcDS@worktop.programming.kicks-ass.net>
References: <PH0PR11MB4839B4D2FB8B8D8D52A62C7F9A629@PH0PR11MB4839.namprd11.prod.outlook.com>
 <YvzJTxOwmikAlZ6j@worktop.programming.kicks-ass.net>
 <20220818011045.v4baekgxajylqxvh@treble>
 <Yv3qHNGBM2aU2NuA@worktop.programming.kicks-ass.net>
 <Yv3sha0zujQxW1X8@xpf.sh.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Yv3sha0zujQxW1X8@xpf.sh.intel.com>
Precedence: bulk

On Thu, Aug 18, 2022 at 03:38:45PM +0800, Pengfei Xu wrote:
>   And this "Missing ENDBR: andw_ax_dx+0x0/0x10 [kvm]" issue was
>   reproduced again after ran syzkaller 5min on ADL-P.

Ha, indeed :/

> > > +/*
> > > + * Create a dummy function pointer reference to prevent objtool from marking
> > > + * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
> > > + * apply_ibt_endbr()).
> > > + */
> > > +#define IBT_NOSEAL(fname)				\
> > > +	".pushsection .discard.endbr.noseal\n\t"	\
> > > +	_ASM_PTR fname "\n\t"				\
> > > +	".popsection\n\t"
> > > +

objtool/check.c:validate_ibt():

                if (!strncmp(sec->name, ".discard", 8)                  ||
		    ...)
		    continue;

So yeah, that'll just get ignored. So at the very least we need to
special case this section.