linux-kernel - RE: [RFC PATCH 22/32] x86/fred: FRED initialization code

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <BN6PR1101MB2161C51C6068026D9C442460A8EB9@BN6PR1101MB2161.namprd11.prod.outlook.com>
Date:   Wed, 21 Dec 2022 05:28:52 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [RFC PATCH 22/32] x86/fred: FRED initialization code

> > >> +	wrmsrl(MSR_IA32_FRED_STKLVLS,
> > >> +	       FRED_STKLVL(X86_TRAP_DB,  1) |
> > >> +	       FRED_STKLVL(X86_TRAP_NMI, 2) |
> > >> +	       FRED_STKLVL(X86_TRAP_MC,  2) |
> > >> +	       FRED_STKLVL(X86_TRAP_DF,  3));
> > >> +
> > >> +	/* The FRED equivalents to IST stacks... */
> > >> +	wrmsrl(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(DB));
> > >> +	wrmsrl(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(NMI));
> > >> +	wrmsrl(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(DF));
> > > Not quite.. IIRC fred only switches to another stack when the level
> > > of the exception is higher. Specifically, if we trigger #DB while
> > > inside #NMI we will not switch to the #DB stack (since 1 < 2).

Yes, current stack level can only grow higher.

> >
> > There needs to be a new stack for #DF, and just possibly one for #MC.
> > NMI and #DB do not need separate stacks under FRED.
> 
> True, there is very little need to use additional stacks with FRED.

Pretty much.

#DB/NMI from a ring 3 context uses CSL 0, and their CSLs increase only
when happening from a ring 0 context.

> 
> > > Now, as mentioned elsewhere, it all nests a lot saner, but stack
> > > exhaustion is still a thing, given the above, what happens when a
> > > #DB hits an #NMI which tickles a #VE or something?
> > >
> > > I don't think we've increased the exception stack size, but perhaps
> > > we should for FRED?
> >
> > Not sure if it matters too much - it doesn't seem usefully different
> > to IDT delivery.  #DB shouldn't get too deep, and NMI gets properly
> > inhibited now.
> 
> Both #DB and #NMI can end up in perf, and all that goes quite deep :/

Can you please elaborate it a bit?