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]

Date:   Tue, 20 Dec 2022 11:02:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>
Cc:     Xin Li <xin3.li@...el.com>,
        "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>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "ravi.v.shankar@...el.com" <ravi.v.shankar@...el.com>
Subject: Re: [RFC PATCH 22/32] x86/fred: FRED initialization code

On Tue, Dec 20, 2022 at 09:55:31AM +0000, Andrew Cooper wrote:
> On 20/12/2022 9:45 am, Peter Zijlstra wrote:
> > On Mon, Dec 19, 2022 at 10:36:48PM -0800, Xin Li wrote:
> >
> >> +	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).
> 
> 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.

> > 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 :/