From: Andy Lutomirski <luto@amacapital.net>
To: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc: Jann Horn <jannh@google.com>,
the arch/x86 maintainers <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
kernel list <linux-kernel@vger.kernel.org>,
linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
linux-arch <linux-arch@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@kernel.org>,
Balbir Singh <bsingharora@gmail.com>,
Borislav Petkov <bp@alien8.de>,
Cyrill Gorcunov <gorcunov@gmail.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Eugene Syromiatnikov <esyr@redhat.com>,
Florian Weimer <fweimer@redhat.com>,
"H.J. Lu" <hjl.tools@gmail.com>, Jonathan Corbet <corbet@lwn.net>,
Kees Cook <keescook@chromium.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Nadav Amit <nadav.amit@gmail.com>,
Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
Peter Zijlstra <peterz@infradead.org>,
Randy Dunlap <rdunlap@infradead.org>,
"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
Dave Martin <Dave.Martin@arm.com>,
Weijiang Yang <weijiang.yang@intel.com>
Subject: Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET
Date: Wed, 2 Sep 2020 21:35:56 -0700 [thread overview]
Message-ID: <40BC093A-F430-4DCC-8DC0-2BA90A6FC3FA@amacapital.net> (raw)
In-Reply-To: <46e42e5e-0bca-5f3f-efc9-5ab15827cc0b@intel.com>
> On Sep 2, 2020, at 7:53 PM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>
> On 9/2/2020 4:50 PM, Andy Lutomirski wrote:
>>>> On Sep 2, 2020, at 3:13 PM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>>>
>>> On 9/2/2020 1:03 PM, Jann Horn wrote:
>>>>> On Tue, Aug 25, 2020 at 2:30 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>>>> Add REGSET_CET64/REGSET_CET32 to get/set CET MSRs:
>>>>>
>>>>> IA32_U_CET (user-mode CET settings) and
>>>>> IA32_PL3_SSP (user-mode Shadow Stack)
>>>> [...]
>>>>> diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
>>>> [...]
>>>>> +int cetregs_get(struct task_struct *target, const struct user_regset *regset,
>>>>> + struct membuf to)
>>>>> +{
>>>>> + struct fpu *fpu = &target->thread.fpu;
>>>>> + struct cet_user_state *cetregs;
>>>>> +
>>>>> + if (!boot_cpu_has(X86_FEATURE_SHSTK))
>>>>> + return -ENODEV;
>>>>> +
>>>>> + fpu__prepare_read(fpu);
>>>>> + cetregs = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>>>>> + if (!cetregs)
>>>>> + return -EFAULT;
>>>> Can this branch ever be hit without a kernel bug? If yes, I think
>>>> -EFAULT is probably a weird error code to choose here. If no, this
>>>> should probably use WARN_ON(). Same thing in cetregs_set().
>>>
>>> When a thread is not CET-enabled, its CET state does not exist. I looked at EFAULT, and it means "Bad address". Maybe this can be ENODEV, which means "No such device"?
Having read the code, I’m unconvinced. It looks like a get_xsave_addr() failure means “state not saved; task sees INIT state”. So *maybe* it’s reasonable -ENODEV this, but I’m not really convinced. I tend to think we should return the actual INIT state and that we should permit writes and handle them correctly.
Dave, what do you think?
>>>
>>> [...]
>>>
>>>>> @@ -1284,6 +1293,13 @@ static struct user_regset x86_32_regsets[] __ro_after_init = {
>>>> [...]
>>>>> + [REGSET_CET32] = {
>>>>> + .core_note_type = NT_X86_CET,
>>>>> + .n = sizeof(struct cet_user_state) / sizeof(u64),
>>>>> + .size = sizeof(u64), .align = sizeof(u64),
>>>>> + .active = cetregs_active, .regset_get = cetregs_get,
>>>>> + .set = cetregs_set
>>>>> + },
>>>>> };
>>>> Why are there different identifiers for 32-bit CET and 64-bit CET when
>>>> they operate on the same structs and have the same handlers? If
>>>> there's a good reason for that, the commit message should probably
>>>> point that out.
>>>
>>> Yes, the reason for two regsets is that fill_note_info() does not expect any holes in a regsets. I will put this in the commit log.
>>>
>>>
>> Perhaps we could fix that instead?
>
> As long as we understand the root cause, leaving it as-is may be OK.
The regset mechanism’s interactions with compat are awful. Let’s please not make it worse. One CET regret is good; two is not good.
>
> I had a patch in the past, but did not follow up on it.
>
> https://lore.kernel.org/lkml/20180717162502.32274-1-yu-cheng.yu@intel.com/
>
> Yu-cheng
next prev parent reply other threads:[~2020-09-03 4:36 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-25 0:26 [PATCH v11 0/9] Control-flow Enforcement: Indirect Branch Tracking, PTRACE Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 1/9] x86/cet/ibt: Add Kconfig option for user-mode Indirect Branch Tracking Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 2/9] x86/cet/ibt: User-mode Indirect Branch Tracking support Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 3/9] x86/cet/ibt: Handle signals for Indirect Branch Tracking Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 4/9] x86/cet/ibt: ELF header parsing " Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 5/9] x86/cet/ibt: Update arch_prctl functions " Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET Yu-cheng Yu
2020-09-02 20:03 ` Jann Horn
2020-09-02 22:13 ` Yu, Yu-cheng
2020-09-02 23:50 ` Andy Lutomirski
2020-09-03 2:53 ` Yu, Yu-cheng
2020-09-03 4:35 ` Andy Lutomirski [this message]
2020-09-03 14:26 ` Dave Hansen
2020-09-03 14:54 ` Andy Lutomirski
2020-09-03 16:09 ` Yu, Yu-cheng
2020-09-03 16:11 ` Dave Hansen
2020-09-03 16:15 ` Andy Lutomirski
2020-09-03 16:25 ` Dave Hansen
2020-09-03 16:32 ` Andy Lutomirski
2020-09-03 16:42 ` Dave Hansen
2020-09-03 17:59 ` Yu, Yu-cheng
2020-09-03 16:21 ` Yu, Yu-cheng
2020-09-03 16:25 ` Andy Lutomirski
2020-09-03 0:33 ` Jann Horn
2020-09-03 2:53 ` Yu, Yu-cheng
2020-08-25 0:26 ` [PATCH v11 7/9] x86/vdso/32: Add ENDBR32 to __kernel_vsyscall entry point Yu-cheng Yu
2020-08-25 0:26 ` [PATCH v11 8/9] x86/vdso: Insert endbr32/endbr64 to vDSO Yu-cheng Yu
2020-08-25 0:33 ` Andy Lutomirski
2020-08-25 16:13 ` Yu, Yu-cheng
2020-08-25 0:26 ` [PATCH v11 9/9] x86: Disallow vsyscall emulation when CET is enabled Yu-cheng Yu
2020-08-25 0:32 ` Andy Lutomirski
2020-08-25 9:14 ` Florian Weimer
2020-08-25 15:08 ` Yu, Yu-cheng
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