linux-kernel - RE: the x86 sysret_rip test fails on the Intel FRED architecture

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Message-ID: <SA1PR11MB6734368CB5479E2D1C03C978A8CB9@SA1PR11MB6734.namprd11.prod.outlook.com>
Date:   Sun, 22 Jan 2023 03:01:53 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Brian Gerst <brgerst@...il.com>,
        "Hansen, Dave" <dave.hansen@...el.com>
CC:     "H. Peter Anvin" <hpa@...or.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: the x86 sysret_rip test fails on the Intel FRED architecture

> > >> If not intentional, it might be something that can still be fixed.
> > >> If it is intentional and is going to be with us for a while we have
> > >> a few options.  If userspace is _really_ depending on this
> > >> behavior, we could just clobber r11 ourselves in the FRED entry
> > >> path.  If not, we can remove the assertion in the selftest.
> > > We can't clobber it in the FRED entry path, since it is common for
> > > all events, but we could do it in the syscall dispatch.
> > >
> > > However, it doesn't seem to make sense to do so to me. The current
> > > behavior is much more of an artifact than desired behavior.
> > I guess the SDM statements really are for the kernel's benefit and not
> > for userspace.  Userspace _should_ be treating SYSCALL like a CALL and
> > r11 like any old register that can be clobbered.  Right now, the
> > kernel just happens to clobber it with RFLAGS.
> >
> > I do the the odds of anyone relying on this behavior are pretty small.
> > Let's just zap the check from the selftest, document what we did in
> > the FRED docs and changelog and move on.
> 
> Keep the selftest check, but also accept preserved RCX/R11.  What really matters is
> that the kernel isn't leaking data.

I feel it the same way, it looks to me that the check is to make sure
R11 doesn't leak any kernel data because the Linux kernel deliberately
overwrites R11 with the value of user level flags just before returning
to user level.

I wanted to zap the check, but as HPA said, this is an artifact to not leak
any kernel data.  I guess it doesn't make a difference if the kernel sets
R11 to 0.

Maybe it's still reasonable to keep such a check for IDT.  However, it makes
no sense for FRED systems, because all GP registers are saved/restored upon
event delivery/return.

Thanks!
  Xin

> 
> --
> Brian Gerst