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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 23 Dec 2016 18:11:23 -0800
Message-ID: <CALCETrUW+zRnYT0oG_cwkn3xOjpTgKVtT71GeA2JucTkF5+Log@mail.gmail.com>
Subject: Re: [v2 6/7] x86/traps: Fixup general protection faults caused by UMIP
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, linux-msdos@vger.kernel.org,
        wine-devel@winehq.org, Andrew Morton <akpm@linux-foundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Brian Gerst <brgerst@gmail.com>,
        Chen Yucong <slaoub@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>, Huang Rui <ray.huang@amd.com>,
        Jiri Slaby <jslaby@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>,
        Shuah Khan <shuah@kernel.org>, Vlastimil Babka <vbabka@suse.cz>,
        Tony Luck <tony.luck@intel.com>, Paolo Bonzini <pbonzini@redhat.com>,
        "Liang Z . Li" <liang.z.li@intel.com>,
        Alexandre Julliard <julliard@winehq.org>, Stas Sergeev <stsp@list.ru>
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On Fri, Dec 23, 2016 at 5:37 PM, Ricardo Neri
<ricardo.neri-calderon@linux.intel.com> wrote:
> If the User-Mode Instruction Prevention CPU feature is available and
> enabled, a general protection fault will be issued if the instructions
> sgdt, sldt, sidt, str or smsw are executed from user-mode context
> (CPL > 0). If the fault was caused by any of the instructions protected
> by UMIP, fixup_umip_exceptino will emulate dummy results for these
> instructions.
>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Chen Yucong <slaoub@gmail.com>
> Cc: Chris Metcalf <cmetcalf@mellanox.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Huang Rui <ray.huang@amd.com>
> Cc: Jiri Slaby <jslaby@suse.cz>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Liang Z. Li <liang.z.li@intel.com>
> Cc: Alexandre Julliard <julliard@winehq.org>
> Cc: Stas Sergeev <stsp@list.ru>
> Cc: x86@kernel.org
> Cc: linux-msdos@vger.kernel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
>  arch/x86/kernel/traps.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index bf0c6d0..5044fb3 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -64,6 +64,7 @@
>  #include <asm/trace/mpx.h>
>  #include <asm/mpx.h>
>  #include <asm/vm86.h>
> +#include <asm/umip.h>
>
>  #ifdef CONFIG_X86_64
>  #include <asm/x86_init.h>
> @@ -491,6 +492,9 @@ do_general_protection(struct pt_regs *regs, long error_code)
>         RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
>         cond_local_irq_enable(regs);
>
> +       if (user_mode(regs) && !fixup_umip_exception(regs))
> +               return;
> +

I would do fixup_umip_exception(regs) == 0 to make it more obvious
what's going on.

Also, since you're allowing this in v8086 mode, I think this should
have an explicit test in
tools/testing/selftests/x86/entry_from_vm86.c.  I *think* it will work
fine, but the pt_regs handling in vm86 mode is quite scary and has
been rewritten recently.