linux-kernel - Re: [PATCH v27 06/10] x86/cet/ibt: Update arch_prctl functions for Indirect Branch Tracking

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Date:   Tue, 20 Jul 2021 10:09:43 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "Xu, Pengfei" <pengfei.xu@...el.com>,
        "vedvyas.shanbhogue@...el.com" <vedvyas.shanbhogue@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "nadav.amit@...il.com" <nadav.amit@...il.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "corbet@....net" <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v27 06/10] x86/cet/ibt: Update arch_prctl functions for
 Indirect Branch Tracking

On 7/19/2021 11:21 AM, Edgecombe, Rick P wrote:
> On Fri, 2021-05-21 at 15:15 -0700, Yu-cheng Yu wrote:
>> From: "H.J. Lu" <hjl.tools@...il.com>
>>
>> Update ARCH_X86_CET_STATUS and ARCH_X86_CET_DISABLE for Indirect
>> Branch
>> Tracking.
>>
>> Signed-off-by: H.J. Lu <hjl.tools@...il.com>
>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
>> Reviewed-by: Kees Cook <keescook@...omium.org>
>> ---
>>   arch/x86/kernel/cet_prctl.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/arch/x86/kernel/cet_prctl.c
>> b/arch/x86/kernel/cet_prctl.c
>> index b426d200e070..bd3c80d402e7 100644
>> --- a/arch/x86/kernel/cet_prctl.c
>> +++ b/arch/x86/kernel/cet_prctl.c
>> @@ -22,6 +22,9 @@ static int cet_copy_status_to_user(struct
>> thread_shstk *shstk, u64 __user *ubuf)
>>                  buf[2] = shstk->size;
>>          }
>>   
>> +       if (shstk->ibt)
>> +               buf[0] |= GNU_PROPERTY_X86_FEATURE_1_IBT;
>> +
> Can you have IBT enabled but not shadow stack via kernel parameters?
> Outside this diff it has:
> if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> 	return -ENOTSUPP;

If shadow stack is disabled by the kernel parameter, IBT is also disabled.

> So if "no_user_shstk" is set, this can't be used for IBT. But the
> kernel would attempt to enable IBT.

It will not.

> Also if so, the CR4 bit enabling logic needs adjusting in this IBT
> series. If not, we should probably mention this in the docs and enforce
> it. It would then follow the logic in Kconfig, so maybe the simplest.
> Like maybe instead of no_user_shstk, just no_user_cet?

If shadow stack is disabled (from either Kconfig or kernel 
command-line), then IBT is also disabled.  However, we still need two 
kernel parameters because no_user_ibt can be useful sometimes.  I will 
add a sentence in the document to indicate that IBT depends on shadow 
stack.

Thanks,
Yu-cheng