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From: hpa@zytor.com
To: Marco Elver <elver@google.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>
Subject: Re: [PATCH 2/3] x86: Move CPU feature test out of uaccess region
Date: Fri, 31 May 2019 16:41:46 -0700	[thread overview]
Message-ID: <3B49EF08-147F-451C-AA5B-FC4E1B8568EE@zytor.com> (raw)
In-Reply-To: <CANpmjNOsPnVd50cTzUW8UYXPGqpSnRLcjj=JbZraTYVq1n18Fw@mail.gmail.com>

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.

  reply	other threads:[~2019-06-01  1:14 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-29 14:14 [PATCH v2 0/3] Bitops instrumentation for KASAN Marco Elver
2019-05-29 14:14 ` [PATCH v2 1/3] lib/test_kasan: Add bitops tests Marco Elver
2019-05-29 15:15   ` Mark Rutland
2019-05-29 14:15 ` [PATCH 2/3] x86: Move CPU feature test out of uaccess region Marco Elver
2019-05-29 14:29   ` hpa
2019-05-31  9:57     ` Marco Elver
2019-05-31 23:41       ` hpa [this message]
2019-06-03  9:03         ` Marco Elver
2019-05-29 14:15 ` [PATCH v2 3/3] asm-generic, x86: Add bitops instrumentation for KASAN Marco Elver
2019-05-29 15:32   ` Mark Rutland
2019-05-29 15:40     ` Marco Elver
2019-05-31 15:12 ` [PATCH v2 0/3] Bitops " Marco Elver

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