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From:   Marco Elver <elver@google.com>
Date:   Mon, 3 Jun 2019 11:03:30 +0200
Message-ID: <CANpmjNMt8QK+j6yo8ut1UNe0wS3_B4iqG5N_eTmJcWj4TpZaDQ@mail.gmail.com>
Subject: Re: [PATCH 2/3] x86: Move CPU feature test out of uaccess region
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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Thanks for the clarification.

I found that static_cpu_has was replaced by static_cpu_has_safe:
https://lkml.org/lkml/2016/1/24/29 -- so is it fair to assume that
both are equally safe at this point?

I have sent a follow-up patch which uses static_cpu_has:
http://lkml.kernel.org/r/20190531150828.157832-3-elver@google.com

Many thanks!
-- Marco

On Sat, 1 Jun 2019 at 03:13, <hpa@zytor.com> wrote:
>
> On May 31, 2019 2:57:36 AM PDT, Marco Elver <elver@google.com> wrote:
> >On Wed, 29 May 2019 at 16:29, <hpa@zytor.com> wrote:
> >>
> >> On May 29, 2019 7:15:00 AM PDT, Marco Elver <elver@google.com> wrote:
> >> >This patch is a pre-requisite for enabling KASAN bitops
> >> >instrumentation:
> >> >moves boot_cpu_has feature test out of the uaccess region, as
> >> >boot_cpu_has uses test_bit. With instrumentation, the KASAN check
> >would
> >> >otherwise be flagged by objtool.
> >> >
> >> >This approach is preferred over adding the explicit kasan_check_*
> >> >functions to the uaccess whitelist of objtool, as the case here
> >appears
> >> >to be the only one.
> >> >
> >> >Signed-off-by: Marco Elver <elver@google.com>
> >> >---
> >> >v1:
> >> >* This patch replaces patch: 'tools/objtool: add kasan_check_* to
> >> >  uaccess whitelist'
> >> >---
> >> > arch/x86/ia32/ia32_signal.c | 9 ++++++++-
> >> > 1 file changed, 8 insertions(+), 1 deletion(-)
> >> >
> >> >diff --git a/arch/x86/ia32/ia32_signal.c
> >b/arch/x86/ia32/ia32_signal.c
> >> >index 629d1ee05599..12264e3c9c43 100644
> >> >--- a/arch/x86/ia32/ia32_signal.c
> >> >+++ b/arch/x86/ia32/ia32_signal.c
> >> >@@ -333,6 +333,7 @@ int ia32_setup_rt_frame(int sig, struct ksignal
> >> >*ksig,
> >> >       void __user *restorer;
> >> >       int err = 0;
> >> >       void __user *fpstate = NULL;
> >> >+      bool has_xsave;
> >> >
> >> >       /* __copy_to_user optimizes that into a single 8 byte store
> >*/
> >> >       static const struct {
> >> >@@ -352,13 +353,19 @@ int ia32_setup_rt_frame(int sig, struct
> >ksignal
> >> >*ksig,
> >> >       if (!access_ok(frame, sizeof(*frame)))
> >> >               return -EFAULT;
> >> >
> >> >+      /*
> >> >+       * Move non-uaccess accesses out of uaccess region if not
> >strictly
> >> >+       * required; this also helps avoid objtool flagging these
> >accesses
> >> >with
> >> >+       * instrumentation enabled.
> >> >+       */
> >> >+      has_xsave = boot_cpu_has(X86_FEATURE_XSAVE);
> >> >       put_user_try {
> >> >               put_user_ex(sig, &frame->sig);
> >> >               put_user_ex(ptr_to_compat(&frame->info),
> >&frame->pinfo);
> >> >               put_user_ex(ptr_to_compat(&frame->uc), &frame->puc);
> >> >
> >> >               /* Create the ucontext.  */
> >> >-              if (boot_cpu_has(X86_FEATURE_XSAVE))
> >> >+              if (has_xsave)
> >> >                       put_user_ex(UC_FP_XSTATE,
> >&frame->uc.uc_flags);
> >> >               else
> >> >                       put_user_ex(0, &frame->uc.uc_flags);
> >>
> >> This was meant to use static_cpu_has(). Why did that get dropped?
> >
> >I couldn't find any mailing list thread referring to why this doesn't
> >use static_cpu_has, do you have any background?
> >
> >static_cpu_has also solves the UACCESS warning.
> >
> >If you confirm it is safe to change to static_cpu_has(), I will change
> >this patch. Note that I should then also change
> >arch/x86/kernel/signal.c to mirror the change for 32bit  (although
> >KASAN is not supported for 32bit x86).
> >
> >Thanks,
> >-- Marco
>
> I believe at some point the intent was that boot_cpu_has() was safer and could be used everywhere.
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.