linux-kernel - Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of access

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Date:   Wed, 29 Mar 2023 01:54:37 -0400
From:   Gregory Price <gregory.price@...verge.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Gregory Price <gourry.memverge@...il.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Linux-Arch <linux-arch@...r.kernel.org>, avagin@...il.com,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, krisman@...labora.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>, shuah <shuah@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>, tongtiangen@...wei.com,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of
 access_ok

On Wed, Mar 29, 2023 at 07:58:51PM +0200, Oleg Nesterov wrote:
> On 03/29, Gregory Price wrote:
> >
> > On Wed, Mar 29, 2023 at 07:13:22PM +0200, Oleg Nesterov wrote:
> > >
> > > -		if (selector && !access_ok(selector, sizeof(*selector)))
> > > -			return -EFAULT;
> > > -
> > >  		break;
> > >  	default:
> > >  		return -EINVAL;
> > >
> >
> > The result of this would be either a task calling via prctl or a tracer
> > calling via ptrace would be capable of setting selector to a bad pointer
> > and producing a SIGSEGV on the next system call.
> 
> Yes,
> 
> > It's a pretty small footgun, but maybe that's reasonable?
> 
> I hope this is reasonable,
> 
> > From a user perspective, debugging this behavior would be nightmarish.
> > Your call to prctl/ptrace would succeed and the process would continue
> > to execute until the next syscall - at which point you incur a SIGSEGV,
> 
> Yes. But how does this differ from the case when, for example, user
> does prtcl(PR_SET_SYSCALL_USER_DISPATCH, selector = 1) ? Or another
> bad address < TASK_SIZE?
> 
> access_ok() will happily succeed, then later syscall_user_dispatch()
> will equally trigger SIGSEGV.
> 
> Oleg.
> 

I'm convinced now, this feels like the correct solution.  I will pull
your suggested patch ahead and drop the task variant of access_ok.

Am I ok to add your signed-off-by to the suggested patch, and i'll add
it to the series?  Not quite sure what the correct set of tags is,
since i don't have any suggested changes to your patch.

~Gregory