linux-kernel - Re: [RFC PATCH 06/13] x86/uintr: Introduce uintr receiver syscalls

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Message-ID: <87a6k2g7mo.ffs@tglx>
Date:   Fri, 24 Sep 2021 02:05:51 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Sohil Mehta <sohil.mehta@...el.com>
Cc:     x86@...nel.org, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <christian@...uner.io>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Ashok Raj <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Zeng Guang <guang.zeng@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Randy E Witt <randy.e.witt@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ramesh Thomas <ramesh.thomas@...el.com>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [RFC PATCH 06/13] x86/uintr: Introduce uintr receiver syscalls

On Thu, Sep 23 2021 at 14:26, Greg KH wrote:
> On Mon, Sep 13, 2021 at 01:01:25PM -0700, Sohil Mehta wrote:
>> +SYSCALL_DEFINE2(uintr_register_handler, u64 __user *, handler, unsigned int, flags)
>> +{
>> +	int ret;
>> +
>> +	if (!uintr_arch_enabled())
>> +		return -EOPNOTSUPP;
>> +
>> +	if (flags)
>> +		return -EINVAL;
>> +
>> +	/* TODO: Validate the handler address */
>> +	if (!handler)
>> +		return -EFAULT;
>
> Um, that's a pretty big "TODO" here.
>
> How are you going to define what is, and what is not, an allowed
> "handler"?

The requirement is obviously that this is a valid user space address,
but that's so hard to validate that it needs to be done later.

At least the documentation claims that a non user space address should
result in a #GP on delivery. Whether that holds in all corner cases (see
the spurious handling muck) is a different question and might come back
to us later through a channel which we hate with a passion :)

> I'm sure the security people would like to get involved here, as well as
> the auditing people.  Have you talked with them about their requirements
> for this type of stuff?

The handler is strictly a user space address and user space is generally
allowed to shoot itself into the foot. If the address is bogus then this
will resolve into inaccessible, not-mapped or not exectuable space and
the application can keep the pieces.

Whether the hardware handles the resulting exception correctly is a
different question, but that can't be prevented by any sanity check on
the address at registration time.

Thanks,

        tglx