linux-kernel - Re: [PATCH 0/3] FSGSBASE fix, test, and a semi-related cleanup

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Date:   Mon, 1 Jul 2019 20:57:12 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 0/3] FSGSBASE fix, test, and a semi-related cleanup

On Mon, Jul 1, 2019 at 8:43 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> In -tip, if FSGSBASE and PTI are on, the kernel crashes if SYSENTER
> happens with TF set.  It also crashes under if a non-NMI paranoid
> entry happens for any other reason from kernel mode with user GSBASE
> and user CR3, e.g. due to MOV SS shenanigans.
>
> This series fixes the bug.  It also adds another test to make sure
> we exercise SYSENTER with TF set regardless of what vendor's CPU
> we're on, although the test isn't needed to detect the bug: the
> single_step_syscall_32 and mov_ss_trap_* tests also trigger it.  And
> it compiles ignore_sysret out on IA32_EMULATION kernels -- I wasted
> a couple minutes while debugging this wondering whether I was
> accidentally triggering ignore_sysret.

I forgot to mention: even with this applied, the x86/cpu tree is not
ready for prime time.  The fsgsbase test case fails on released
kernels and crashes on x86/cpu.  I haven't gotten to the bottom of it
yet.  The test code looks a bit dubious, but that doesn't necessarily
mean the kernel is okay.