From:   Thomas Gleixner <tglx@linutronix.de>
To:     Gregory Price <gregory.price@memverge.com>
Cc:     Gregory Price <gourry.memverge@gmail.com>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        oleg@redhat.com, avagin@gmail.com, peterz@infradead.org,
        luto@kernel.org, krisman@collabora.com, corbet@lwn.net,
        shuah@kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH v13 1/3] syscall_user_dispatch: helper function to
 operate on given task
In-Reply-To: <ZBnhmtvlenY15P32@memverge.com>
References: <20230301205843.2164-1-gregory.price@memverge.com>
 <20230301205843.2164-2-gregory.price@memverge.com> <87cz529kni.ffs@tglx>
 <ZBnhmtvlenY15P32@memverge.com>
Date:   Tue, 21 Mar 2023 20:46:26 +0100
Message-ID: <87a605anvx.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Tue, Mar 21 2023 at 12:55, Gregory Price wrote:
> On Tue, Mar 21, 2023 at 04:41:37PM +0100, Thomas Gleixner wrote:
>> On Wed, Mar 01 2023 at 15:58, Gregory Price wrote:
>> > +static int task_set_syscall_user_dispatch(struct task_struct *task, unsigned long mode,
>> > +					  unsigned long offset, unsigned long len,
>> > +					  char __user *selector)
>> >  {
>> >  	switch (mode) {
>> >  	case PR_SYS_DISPATCH_OFF:
>>         ...
>> 
>> 	case PR_SYS_DISPATCH_ON:
>> 		if (selector && !access_ok(selector, sizeof(*selector)))
>> 			return -EFAULT;
>> 
>> I'm not seing how this can work on ARM64 when user pointer tagging is
>> enabled in the tracee, but not in the tracer. In such a case, if the
>> pointer is tagged, access_ok() will fail because access_ok() wont untag
>> it.
>
> I see that untagged_addr(x) is available to clear tags, I don't see an
> immediate issues with converting to:
>
> !access_ok(untagged_addr(selector), sizeof(*selector))

If this would be correct, then access_ok() on arm64 would
unconditionally untag the checked address, but it does not. Simply
because untagging is only valid if the task enabled pointer tagging. If
it didn't a tagged pointer is obviously invalid.

Why would ptrace make this suddenly valid?

Just because it's in the way of what you want to achieve is not a really
sufficient justification.

Thanks,

        tglx