linux-kernel - Re: [PATCH v13 1/3] syscall_user_dispatch: helper function to operate on given task

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Date:   Tue, 21 Mar 2023 20:46:26 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Gregory Price <gregory.price@...verge.com>
Cc:     Gregory Price <gourry.memverge@...il.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        oleg@...hat.com, avagin@...il.com, peterz@...radead.org,
        luto@...nel.org, krisman@...labora.com, corbet@....net,
        shuah@...nel.org, Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v13 1/3] syscall_user_dispatch: helper function to
 operate on given task

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