linux-kernel - Re: [PATCH 2/3] x86: Move CPU feature test out of uaccess region

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Message-ID: <3B49EF08-147F-451C-AA5B-FC4E1B8568EE@zytor.com>
Date:   Fri, 31 May 2019 16:41:46 -0700
From:   hpa@...or.com
To:     Marco Elver <elver@...gle.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH 2/3] x86: Move CPU feature test out of uaccess region

On May 31, 2019 2:57:36 AM PDT, Marco Elver <elver@...gle.com> wrote:
>On Wed, 29 May 2019 at 16:29, <hpa@...or.com> wrote:
>>
>> On May 29, 2019 7:15:00 AM PDT, Marco Elver <elver@...gle.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@...gle.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.
-- 
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