linux-kernel - Re: [PATCH v1 11/19] x86/sev-es: Convert to insn

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Date:   Mon, 28 Dec 2020 11:15:11 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     kernel test robot <lkp@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        kbuild-all@...ts.01.org, clang-built-linux@...glegroups.com,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 11/19] x86/sev-es: Convert to insn_decode()

On Fri, Dec 25, 2020, Borislav Petkov wrote:
> On Fri, Dec 25, 2020 at 06:50:33PM +0800, kernel test robot wrote:
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kernel test robot <lkp@...el.com>
> > 
> > All warnings (new ones prefixed by >>):
> > 
> > >> arch/x86/kernel/sev-es.c:258:7: warning: variable 'ret' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
> >                    if (!insn_decode_regs(&ctxt->insn, ctxt->regs, buffer, res))
> >                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Yeah, good catch, thanks for reporting.
> 
> Frankly, the readability and "extensiblity" of that function can be
> improved by splitting the two cases (diff ontop):

Alternatively, could the kernel case use insn_decode_regs()?  If
vc_fetch_insn_kernel() were also modified to mirror insn_fetch_from_user(), the
two code paths could be unified except for the the fetch and the PFEC.  E.g.

static int vc_fetch_insn_kernel(struct es_em_ctxt *ctxt,
				unsigned char *buffer)
{
	if (copy_from_kernel_nofault(buffer, (unsigned char *)ctxt->regs->ip, MAX_INSN_SIZE))
		return 0;

	return MAX_INSN_SIZE;
}

static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
{
	char buffer[MAX_INSN_SIZE];
	int nbytes;

	if (user_mode(ctxt->regs))
		nbytes = insn_fetch_from_user(ctxt->regs, buffer);
	else
		nbytes = vc_fetch_insn_kernel(ctxt, buffer);

	if (!nbytes) {
		ctxt->fi.vector     = X86_TRAP_PF;
		ctxt->fi.error_code = X86_PF_INSTR;
		if (user_mode(ctxt->regs))
			ctxt->fi.error_code |= X86_PF_USER;
		ctxt->fi.cr2        = ctxt->regs->ip;
		return ES_EXCEPTION;
	}

	if (!insn_decode_regs(&ctxt->insn, ctxt->regs, buffer, nbytes))
		return ES_DECODE_FAILED;

	return ctxt->insn.immediate.got ? ES_OK : ES_DECODE_FAILED;
}