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Date: Mon, 12 Sep 2016 07:15:16 -0700
From: Kyle Huey <me@...ehuey.com>
To: Borislav Petkov <bp@...e.de>
Cc: "Robert O'Callahan" <robert@...llahan.org>,
linux-api@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Len Brown <len.brown@...el.com>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Huang Rui <ray.huang@....com>,
Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
Andy Lutomirski <luto@...nel.org>,
Juergen Gross <jgross@...e.com>,
Fenghua Yu <fenghua.yu@...el.com>,
"Luis R. Rodriguez" <mcgrof@...nel.org>,
Denys Vlasenko <dvlasenk@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>,
Dmitry Vyukov <dvyukov@...gle.com>,
Paul Gortmaker <paul.gortmaker@...driver.com>,
"Michael S. Tsirkin" <mst@...hat.com>,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Jiri Slaby <jslaby@...e.cz>, Michal Hocko <mhocko@...e.com>,
Alex Thorlton <athorlton@....com>,
Vlastimil Babka <vbabka@...e.cz>,
Mateusz Guzik <mguzik@...hat.com>,
Ben Segall <bsegall@...gle.com>,
John Stultz <john.stultz@...aro.org>,
"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the
CPUID instruction.
Thanks for the review!
On Mon, Sep 12, 2016 at 2:07 AM, Borislav Petkov <bp@...e.de> wrote:
> On Sun, Sep 11, 2016 at 05:29:23PM -0700, Kyle Huey wrote:
>> rr (http://rr-project.org/), a userspace record-and-replay reverse-
>> execution debugger, would like to trap and emulate the CPUID instruction.
>> This would allow us to a) mask away certain hardware features that rr does
>> not support (e.g. RDRAND) and b) enable trace portability across machines
>> by providing constant results.
>>
>> Intel supports faulting on the CPUID instruction in newer processors. Bit
>> 31 of MSR_PLATFORM_INFO advertises support for this feature. It is
>> documented in detail in Section 2.3.2 of
>> http://www.intel.com/content/dam/www/public/us/en/documents/application-notes/virtualization-technology-flexmigration-application-note.pdf.
>>
>> I would like to thank Trevor Saunders <tbsaunde@...aunde.org> for drafting
>> an earlier version of this patch.
>>
>> Signed-off-by Kyle Huey <khuey@...ehuey.com>
>> ---
>> arch/x86/include/asm/msr-index.h | 1 +
>> arch/x86/include/asm/processor.h | 7 ++++
>> arch/x86/include/asm/thread_info.h | 4 +-
>> arch/x86/kernel/process.c | 79 ++++++++++++++++++++++++++++++++++++++
>> include/uapi/linux/prctl.h | 6 +++
>> kernel/sys.c | 12 ++++++
>> 6 files changed, 108 insertions(+), 1 deletion(-)
>
> ...
>
>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>> index 62c0b0e..a189516 100644
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -191,6 +191,76 @@ int set_tsc_mode(unsigned int val)
>> return 0;
>> }
>>
>> +static void hard_disable_CPUID(void)
>
> Why hard_disable? I don't see any soft_disable.
Copied from PR_SET_TSC. Would you prefer something like
disable_cpuid/disable_cpuid_and_set_flag for
hard_disable_CPUID/disable_CPUID?
> Also, I can't say that I like all that screaming "CPUID" :-)
>
> disable_cpuid() looks just fine to me too.
Ok.
>> +{
>> + msr_set_bit(MSR_MISC_FEATURES_ENABLES, 0);
>> +}
>> +
>> +static void disable_CPUID(void)
>> +{
>> + preempt_disable();
>> + if (!test_and_set_thread_flag(TIF_NOCPUID))
>> + /*
>> + * Must flip the CPU state synchronously with
>> + * TIF_NOCPUID in the current running context.
>> + */
>> + hard_disable_CPUID();
>> + preempt_enable();
>> +}
>> +
>> +static void hard_enable_CPUID(void)
>> +{
>> + msr_clear_bit(MSR_MISC_FEATURES_ENABLES, 0);
>> +}
>> +
>> +static void enable_CPUID(void)
>> +{
>> + preempt_disable();
>> + if (test_and_clear_thread_flag(TIF_NOCPUID))
>> + /*
>> + * Must flip the CPU state synchronously with
>> + * TIF_NOCPUID in the current running context.
>> + */
>> + hard_enable_CPUID();
>> + preempt_enable();
>> +}
>> +
>> +static int supports_CPUID_faulting(void)
>> +{
>> + unsigned int lo, hi;
>> +
>> + rdmsr(MSR_PLATFORM_INFO, lo, hi);
>
> rdmsr_safe()
Ok.
>> + if ((lo & (1 << 31)))
>> + return 1;
>> + else
>> + return 0;
>> +}
>>
>> +int get_cpuid_mode(unsigned long adr)
>> +{
>> + unsigned int val;
>> +
>> + if (test_thread_flag(TIF_NOCPUID))
>> + val = PR_CPUID_SIGSEGV;
>> + else
>> + val = PR_CPUID_ENABLE;
>> +
>> + return put_user(val, (unsigned int __user *)adr);
>> +}
>> +
>> +int set_cpuid_mode(unsigned int val)
>> +{
>> + // Only disable/enable_CPUID() if it is supported on this hardware.
>
> Use /* ... */ for comments in the kernel.
Ok.
>> + if (val == PR_CPUID_SIGSEGV && supports_CPUID_faulting())
>> + disable_CPUID();
>> + else if (val == PR_CPUID_ENABLE && supports_CPUID_faulting())
>> + enable_CPUID();
>> + else
>> + return -EINVAL;
>> +
>> + return 0;
>> +}
>> +
>> void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>> struct tss_struct *tss)
>> {
>> @@ -210,6 +280,15 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>> update_debugctlmsr(debugctl);
>> }
>>
>> + if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
>> + test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
>> + /* prev and next are different */
>> + if (test_tsk_thread_flag(next_p, TIF_NOCPUID))
>> + hard_disable_CPUID();
>> + else
>> + hard_enable_CPUID();
>> + }
>> +
>
> Frankly, I can't say that I'm thrilled by this: if this is a niche
> feature which has only a very narrow usage for debugging, I'd much
> prefer if this whole thing were implemented with a static_key which was
> false on the majority of the systems so that __switch_to() tests it much
> cheaply.
>
> Then and only then if your debugger runs arch_prctl(), it would enable
> the key and then set_cpuid_mode() can query the MSR directly instead of
> using another flag in the thread_info flags.
>
> This would keep this niche feature out of the way of the hot paths.
My code is already in the slow path in __switch_to_xtra(), along with
other debugging features like TIF_BLOCKSTEP and TIF_NOTSC. Adding a
bit to the mask tested in __switch_to() shouldn't affect performance
of the hot path.
>> if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
>> test_tsk_thread_flag(next_p, TIF_NOTSC)) {
>> /* prev and next are different */
>> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
>> index a8d0759..641d12b 100644
>> --- a/include/uapi/linux/prctl.h
>> +++ b/include/uapi/linux/prctl.h
>> @@ -197,4 +197,10 @@ struct prctl_mm_map {
>> # define PR_CAP_AMBIENT_LOWER 3
>> # define PR_CAP_AMBIENT_CLEAR_ALL 4
>>
>> +/* Get/set the process' ability to use the CPUID instruction */
>> +#define PR_GET_CPUID 48
>> +#define PR_SET_CPUID 49
>> +# define PR_CPUID_ENABLE 1 /* allow the use of the CPUID instruction */
>> +# define PR_CPUID_SIGSEGV 2 /* throw a SIGSEGV instead of reading the CPUID */
>> +
>> #endif /* _LINUX_PRCTL_H */
>> diff --git a/kernel/sys.c b/kernel/sys.c
>> index 89d5be4..997c6519 100644
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -91,6 +91,12 @@
>> #ifndef SET_TSC_CTL
>> # define SET_TSC_CTL(a) (-EINVAL)
>> #endif
>> +#ifndef GET_CPUID_CTL
>> +# define GET_CPUID_CTL(a) (-EINVAL)
>> +#endif
>> +#ifndef SET_CPUID_CTL
>> +# define SET_CPUID_CTL(a) (-EINVAL)
>> +#endif
>> #ifndef MPX_ENABLE_MANAGEMENT
>> # define MPX_ENABLE_MANAGEMENT() (-EINVAL)
>> #endif
>> @@ -2162,6 +2168,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>> case PR_SET_TSC:
>> error = SET_TSC_CTL(arg2);
>> break;
>> + case PR_GET_CPUID:
>> + error = GET_CPUID_CTL(arg2);
>> + break;
>> + case PR_SET_CPUID:
>> + error = SET_CPUID_CTL(arg2);
>> + break;
>> case PR_TASK_PERF_EVENTS_DISABLE:
>> error = perf_event_task_disable();
>> break;
>
> This whole fun should be in arch_prctl() as it is arch-specific.
Yeah, I was debating about that, and did it this way because of
PR_SET_TSC. Will fix.
> And wherever it ends, it needs documenting in the man page.
Indeed.
Would like to reach consensus on the TIF vs. static key thing before
updating the patch.
Thanks!
- Kyle
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