Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

From: Dave Hansen <dave.hansen@intel.com>
To: Andy Lutomirski <luto@kernel.org>,
	"Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>,
	X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"open list:DOCUMENTATION" <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>,
	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>, Jann Horn <jannh@google.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 v12 8/8] x86: Disallow vsyscall emulation when CET is enabled
Date: Wed, 23 Sep 2020 15:53:46 -0700	[thread overview]
Message-ID: <97273aa8-9aa4-78ee-21ca-d728835f44a7@intel.com> (raw)
In-Reply-To: <CALCETrW7o-j01aWCFjmy2TRm7X75Vd5EW_gAfOSP0nQfekkEmA@mail.gmail.com>

On 9/23/20 3:47 PM, Andy Lutomirski wrote:
> On Wed, Sep 23, 2020 at 3:20 PM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>> On 9/23/2020 3:08 PM, Dave Hansen wrote:
>>> On 9/23/20 3:06 PM, Yu, Yu-cheng wrote:
>>>> I think I'll add a check here for (r + 8) >= TASK_SIZE_MAX.  It is
>>>> better than getting a fault.
>>> There's also wrmsr_safe().
>>>
>> Yes, thanks.
>>
>> Since I am going to change this to:
>>
>> fpu__prepare_write(), then write to the XSAVES area.
>>
>> The kernel does not expect XRSTORS to fail ("Bad FPU state detected..."
>> message).  So maybe still check the address first.
> Surely there are plenty of ways to use ptrace() to poke garbage into
> the FPU state.  We should be able to handle this type of failure
> somewhat gracefully.

Yeah, agreed.  I'd much rather make XRSTORS able to #GP gracefully than
teach the kernel exhaustively about every possible error condition it
can encounter.

We *might* want to do something like to preserve the warning if the task
hasn't been ptrace'd, or had the memory buffer written to directly or
tainted in another way.