linux-kernel - Re: [PATCH] Updating es_em

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Message-ID: <ZzzrDB_1CyNfL0zo@desktop>
Date: Tue, 19 Nov 2024 11:46:20 -0800
From: "Ragavendra B.N." <ragavendra.bn@...il.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, hpa@...or.com, thomas.lendacky@....com,
	ardb@...nel.org, tzimmermann@...e.de, bhelgaas@...gle.com,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Updating es_em_ctxt fi to zero

On Tue, Nov 19, 2024 at 01:26:02PM -0600, Bjorn Helgaas wrote:
> On Tue, Nov 19, 2024 at 10:05:18AM -0800, Ragavendra wrote:
> > Updating es_em_ctxt to zero for the ctxt->fi variable in
> > verify_exception_info when ES_EXCEPTION is returned.
> 
> This commit log basically says in English what the code does in C.  If
> you can include the *reason* why this is important, it will be more
> helpful.  For example, maybe somebody consumes other parts of ctxt.fi
> (a struct es_fault_info), and without this patch, they use junk that
> causes an oops or some other bad thing.
> 
> If the 34ff65901735 Fixes: tag is correct, I suppose the problem
> happens because ctxt is allocated on the stack and contains junk, and
> then svsm_perform_ghcb_protocol() passes it on to
> vc_forward_exception(), which does use fields of ctxt->fi other than
> .vector, which will be junk without this patch.
> 
> Hints and samples for commit logs:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/maintainer-tip.rst?id=v6.11#n134
> 
> Based on "git log --oneline arch/x86/coco/sev", I would expect the
> subject line to have an "x86/sev: " prefix, e.g.,
> 
>   x86/sev: Clear es_em_ctxt.fi to ...
> 
> > Fixes: 34ff65901735 x86/sev: Use kernel provided SVSM Calling Areas
> > Signed-off-by: Ragavendra Nagraj <ragavendra.bn@...il.com>
> > ---
> >  arch/x86/coco/sev/shared.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/arch/x86/coco/sev/shared.c b/arch/x86/coco/sev/shared.c
> > index 71de53194089..b8540d85e6f0 100644
> > --- a/arch/x86/coco/sev/shared.c
> > +++ b/arch/x86/coco/sev/shared.c
> > @@ -239,6 +239,8 @@ static enum es_result verify_exception_info(struct ghcb *ghcb, struct es_em_ctxt
> >  		if ((info & SVM_EVTINJ_VALID) &&
> >  		    ((v == X86_TRAP_GP) || (v == X86_TRAP_UD)) &&
> >  		    ((info & SVM_EVTINJ_TYPE_MASK) == SVM_EVTINJ_TYPE_EXEPT)) {
> > +			memset(&ctxt->fi, 0, sizeof(ctxt->fi));
> > +
> >  			ctxt->fi.vector = v;
> >  
> >  			if (info & SVM_EVTINJ_VALID_ERR)
> > -- 
> > 2.46.1
> > 

Yes Bjorn, I completely agree with the need to update the reason, I will update the commit log and send the newer version accordingly.


--
Thanks & regards,
Ragavendra N