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

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Date:   Sun, 22 Jan 2023 03:38:30 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "H. Peter Anvin" <hpa@...or.com>,
        "Hansen, Dave" <dave.hansen@...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>
CC:     "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

> >> The x86 sysret_rip test has the following assertion:
> >>
> >>         /* R11 and EFLAGS should already match. */
> >>         assert(ctx->uc_mcontext.gregs[REG_EFL] ==
> >>                ctx->uc_mcontext.gregs[REG_R11]);
> >>
> >> This is being tested to avoid kernel state leak due to sysret vs
> >> iret, but that on FRED r11 is *always* preserved, and the test just fails.
> >
> >Let's figure out the reason that FRED acts differently, first.  Right
> >now, the SDM says:
> >
> >	SYSCALL also saves RFLAGS into R11
> >
> >so that behavior of SYSCALL _looks_ architectural to me.  Was this
> >change in SYSCALL behavior with FRED intentional?
> >
> >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.

Yes, adding "regs->r11 = regs->flags" in the SYSCALL dispatch does make
the test pass.

> 
> 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.

We kind of have an agreement that %r11 = %flags after returning from the kernel.

And the question is, is it what we want?