Re: [PATCH v1 11/19] x86/sev-es: Convert to insn_decode()

[Sean Christopherson]

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 <l...@intel.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;
}