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

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Date:   Sat, 21 Jan 2023 16:47:48 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, "Li, Xin3" <xin3.li@...el.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

On Sat, Jan 21, 2023 at 12:34 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 1/20/23 20:59, H. Peter Anvin wrote:
> >> 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.

--
Brian Gerst