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

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Date:   Wed, 29 Mar 2023 19:58:51 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Gregory Price <gregory.price@...verge.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 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.